There's also another element that needs to be explained.MIRV for the Soviets worked the same way -- more targets for less $$$ -- important as there were elements in the USSR which wanted to pull away from mass spending on the arms industries and put more of it into the consumer economy.
Ten years ago on another board now disappeared (it's still there, under a different name, but the original databases have broke); someone raised a big point:
It never ceases to anoy me that the USA had a working ABM system and Congress killed it. The USSR had a working ABM system and it seems that they were prepared to cheat on the ABM treaty.
Reagan always found it a bit of a puzzle as to why the USSR was so against SDI yet they had an ABM system and were looking to expand it.
I think I have finally cracked the reason for the USSR (and later Russia's) anti-ABM stance.
They were barely affording the arms race, which was by 1969-1970, forcing them to:
A.) Build a Strategic Aerospace Defense Force (PVO Strany which was it's own independent branch)
B.) Build a Strategic Bombardment Force (Tu-4, Tu-16, Tu-95, and Tu-160 in Long Range Aviation)
C.) Build a Strategic Rocket Force (Tons of IRBM/ICBMs, all of which again was it's own independent branch)
D.) Build a Ballistic Missile Submarine Force (Yankee/Delta, etc)
E.) Build & Maintain Large Conventional Forces (Air & Ground)
In addition, there were two other factors:
A.) The Soviet Navy was slowly spooling up/agitating for plans to build an all-new Blue Water Navy under Gorshkhov in the 1970s onwards.
B.) Soviet Governmental Ministries were agitating for more money to go to the Civilian side of things -- it's worth noting that in the 1950s or so, various Soviet propaganda had pointed towards the 1970s or thereabouts as when COMMUNISM would be achieved in the USSR, allowing for plenty, etc.
On top of all these factors, there was one BIG thing coming down the road which everyone saw and was afraid of the costs for:
The cyberneticization of society, both military-industrial and civilian -- was accelerating, with a lot of advanced technology products which we would recognize today as "modern" appearing in the decade from 1966 to 1976 (plus or minus a few years left/right):
Integrated Air Defense Networks -- in 1958, the US had the AN/FSQ-7 Computer to run SAGE (Semi Automatic Ground Environment). It had had 1 million lines of code, consumed 1.78 MW of power, weighed 113~ tons and could only conduct 75,000 operations per second on approximately 33K memory and 589K storage (1958) or 287K memory and 633K storage (1961) using some 50,000 vacuum tubes, 170,000 diodes and 703 transistors.
By 1977, the US was introducing the first E-3 AWACS which placed a much more capable SAGE-like environment in a mobile platform - it may interest you to know that version E-16A of the Airborne Operational Computer Program (AOCP) for the AWACS fleet circa 2007 weighed in at 1.2~ million lines of code.
Tank Analog Ballistic Computers -- The first analog ones appeared in 1959 with the M13A1C computer on the M48A2C Patton, followed by German FLER-H in the Leopard 1A4 (1974) and the Russian 1V517 [1В517] on the T-64B (1976) and T-80B (1978).
Tank Digital Ballistic Computers -- The first digital computers entered service in 1975 with the Chieftain Mk 5 upgrade program as part of the Marconi Improved Fire Control System (IFCS); followed in 1979 by the Leopard 2 (EMES-15 FCS) and the US M60A3 (M21 Ballistic Computer) and M1 Abrams (1981). The Soviets trialled the 1V528 Digital Computer in the T-80A prototype in 1978, but it didn't enter service until the 1V528-1 in the T-80U in 1985.
'Modern' Artillery Fire Control -- 1967 with US M18 Field Artillery Digital Computer (FADAC), followed by the US Tactical Fire Direction System (TACFIRE) in 1972.
'Modern' SAMs -- 1975-1978 with the S-300 (SA-10A GRUMBLE) -- PATRIOT dates to this era but was much delayed. Essentially, modern SAMs are "second generation" weapons utilizing digital computers in both the missile and ground systems instead of earlier analog/electromechanical systems.
'Modern' ATGMs - 1972 with US BGM-61 TOW and 1976 in T-64B with 9M112 "Kobra" (AT-8 SONGSTER) plus the Franco-German HOT in 1976 as well. Previous ATGMs were kind of klunky kludges that kind of worked (SAGGER, etc).
Laser Rangefinders - 1970 with the Barr & Stroud LF2 on the Chieftain Mk 3/3 followed by the AN/VVS-1 on the M60A2 (1973) and the 1OP73 [1ОП73] on the T-64B (1976), followed by the T-80B (1978) and T-72A (1979).
Key to all these was the mass production of the new integrated semiconductor circuits -- Project Apollo and the Minuteman program consumed a large portion of US semiconductors while they were going on, but they then left a large and healthy semiconductor industry that the electronics industry could then use.
Meanwhile, the USSR had to import 33 automatic semiconductor slicing machines from Japan in mid-1964 (CIA Intelligence Report 'Production of Electronic Components in the USSR 1958-65') to support increased semiconductor production.
There's a lot of tangential evidence that these imports weren't enough; because we can look "out of order" to the 1980s, when NATO militaries were upgrading existing tanks with digital ballistic computers (Leopards 1A4, 1A5, AMX-30), the Soviets were modernizing the T-55M and T-62M series with the BV-55/BV-62 [БВ-55/БВ-62] series of analog computers from 1983 onwards.
I believe that this dichotomy shows how tight the Soviet economy was cybernetically -- there literally was no spare capacity for digital circuits after:
Prestige Aerospace Programs (Radars, ICBMs, Fighter Jets, Space Program)
Prestige Ground Forces Programs (Radars, ATGMs, high tech tanks [T-64/80])
Prestige Naval Programs (modern missile armed ships consume disproportionate amounts of semiconductors)
Prestige Civilian Programs (i.e. that new supercomputer at the VI Lenin Institute of Physics)
That there was nothing left to digitize the remainder of the Soviet Military, explaining the huge masses of T-55/T-62 having to use analog computers; despite tank ballistic computers being not that computationally intensive (you only need to calculate a single ballistic trajectory and have it ready in about a second).
So how does this all have to do with the ABM Treaty and Missile Defense?
Well, back in 1967-1970ish, the total computing environment to run a SAFEGUARD scale national level defense of the US (and by extension the USSR) was about 20 million instructions per second. That was a LOT back then -- a top of the line IBM System 360 could only do 750 thousand instructions per second; so SAFEGUARD's computer system was totally unique when the design was frozen in 1967.
Now look at this from the Soviet point of view -- they'd just spent the last twenty years from 1950-1970 frantically pacing the US in development of every piece of high tech (Integrated Air Defense System, Bombers, Ballistic Missile Subs, Ballistic Missiles); and now the US was threatening to force another frantic race for parity -- only this time the commodities involved would be increasingly scarce transistors and electronic components that everyone else in the Soviet apparatus wanted.
The Soviet Navy under Gorshkhov wanted modern missile armed combatants with heavy electronics fits to replace the Stalinist era gun cruisers, while at the same time the Civilian-related Ministries wanted small transistor radios and other consumer goods that would prove that Communism Worked (TM).
You can't do all this if all your semiconductor production is going into a very big nationwide ABM build-out of battle management centers and radar systems all over the USSR.
More to the point -- you now have to face the possibility of a steadily thickening ABM shield over the United States which will render your (very expensive) investment in ICBMs obsolete even before you've gotten a decade's worth of service out of them.
Soviet Military Power 1988 laid it out nicely for us, even if it does "inflate" by including "mod" variants as all new systems.

Notice how their major modernization efforts (SS-16, SS-17, SS-18, SS-19) come AFTER the 1970s SALT/ABM Treaties which not only:
A.) Placed limits on US strategic nuclear forces, both missile and bomber based?
B.) Made cheap large area ABM impossible through careful crafting of the ABM treaty (that in itself is a post of it's own) to use physics against ABM system designers trying to stay compliant with the treaty.
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The Soviets were willing to do sneaky "edge case" violations of arms control, banking that we wouldn't politically "overturn the apple cart", namely:
1.) Building a huge radar in violation of the ABM treaty for "something else", but it's totally not an ABM radar! Honest Tovarisch!
2.) Putting several DELTA SSBNs to sea on trials/in service, breaking the SALT limits on SSBN force structure, by claiming that several YANKEE SSBNs were "scheduled for retirement" yet...they were mysteriously "fully armed and operational"
3.) Building the SA-12B GIANT (S-300V with Large 98M2 Missile) circa 1983. The large 98M2 misile had a 2.5 km/sec VBo; really close to the limit of 3 km/sec for SAMs that was agreed to under the 1972 Common Understanding Related to the First Agreed Statement (regarding the ABM Treaty).
Now, in some cases we did do push back -- THAAD started out as a low key response to the Soviet development of S-300V; and it got "important enough" that the following systems were named specifically in "The 1997 Agreement on Confidence Building Measures regarding Tactical ABM":
USA
Theater High-Altitude Area Defense (THAAD) System
Navy Theater-Wide Theater Ballistic Missile Defense Program
Russia (and successors like Belarus and Ukraine)
S-300V system, known to the United States of America as the SA-12
But overall, they weren't willing to commit absolute outright blatant treaty violations, because they knew that if they did so, we'd detect it via satellites and we'd immediately engage them in an unrestrained arms race, which would then cause the Soviet economy to collapse within 15 years.