So where's the more modern stuff? Being saved for something else, or do they just not have it in sensible quantities?
They probably don't have much of it, and what is in existence is probably largely being kept for defence.
A lot of Russia's newest and greatest hardware is only available in very very small numbers, enough to roll out for the odd demonstration, not in war fighting quantities.
Given how well maintained the stuff that has been seen seems to be, I wouldn't be surprised if a lot of the newer stuff is simply not working, as the stuff from the 70's and 80's is likely far less complicated and hardier in terms of maintenance requirements than more modern equipment that tends to need a higher level of technical training to maintain. and often requires more expensive and harder to produce parts that are replaced more often.
An old diesel engine for example can often be started after being left for years if you simply change out the fuel, a modern higher performance one might have issues from just a few months in storage if it's not been prepped properly due to both natural degradation of them (IE seals and pipes that require oil or other fluids passing over them), and things like mother natures various saboteurs* which have a tendency to find all the little nooks, crannies and various paper and rubber bits ideal for either nests or just plain tasty.
Simple things like tyres need to be looked after, and if that's not happening as has been suggested, then it doesn't bode well for higher tech stuff that requires more than telling a conscript "drive that truck around the parking area a couple of times and then press these two buttons".
*IIRC rats, mice, various insects, and in Germany there is a real issue with Weasels for example (as a german car owner you can buy weasel insurance to cover when they get at your car...).