Soviet Union still had WWII era t-34/85 and Su-100s in inventory in the late 1980s. Soviet Union had the habit of never throwing ancient equipment away even if they are likely to function as nothing more than death traps for their own crews. An extreme example is Soviet Union continued to maintain pre-WWI era 12 inch coastal gun turrets from the czarist era all the way until 1990s. I think this is because there is always the possibility that when higher quality forces on both sides are spent, opportunity can still arise to allow tactical advantage to be gained at crucial moment by rushing in low quality forces and be willing absorbing much higher casualties than the enemy.
I think the Chinese military probably also embody this thinking in its traditional doctrine.
In the west, overt preparations for such approaches might be considered bad for morale because it would seem to suggest planing staff value soldiers' lives less than the soldiers themselves might.
Everyone does this, take famous USAF bone yard as an example.
Rather than merely scrap old planes, they are mothballed and ostensibly kept for spares, but in fact, they are as much a war reserve as a source of spare parts for planes still flying.
The biggest difference is that the Chinese and Russians keep their old stuff operational in low tier units, while the US just mothballs them and buys new kit for everyone.
How much clout and influence the arms manufactures have with governments actually plays a bit part in this decision making process.
It actually makes very good sense to keep the older stuff active with low tier units to keep the knowledge and experience of their operations current, and also to keep the factories making spares and overhaul facilities for these legacy weapons open. Being able to buy fewer examples of expensive new kit would also be a big plus.
Obviously, how likely you see yourself as needing to call up these war reserves is also a major factor in the decision making process.
For the US, I think its fair to say few in the military, never mind political leadership, would ever think it necessary for them to have to re-activate the war reserves out of operational necessity. However, if they ever find themselves having to call up that reserve, they may well find that those war reserves are nowhere near as useful as they think because of the total discontinuation in use.
I would dare say most current active US military pilots are going to have a hard enough time adapting to even early 90s versions of the planes they are currently flying, never mind older types with their analogue instruments, lack of FBW and primitive sensors and weapons. The ground crews will probably have a harder time working with vacuum tubes and non-modular designed circuitry. Not least because most of those parts would have long been out of production.