I believe the Soviets/Russians has some serious issues with the long term stability of their solid fuel rocket propellant. I tried looking to see why the rail mobile SS-24's were withdrawn and it would seem that something may have been amiss there. On top of that, the SS-N-20 in the Typhoon class had extensive issues during development with the motors and the follow on developments did as well.
It's possible some of the ICBM and SLBM fleet simply didn't work and were abandoned or destroyed as a result.
-Dave
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