I don’t have trouble believing they sent all that stuff in, that’s how Russians operate. They never throw anything away.
But it’s not their oldest stuff. Their principal tank loss, by the hundred, is the T72BV3 mod 2016. And there hundreds of T-90’s committed, perhaps over half of those in their inventory.
Nor are the men their worst soldiers. In most cases they are their best. Most of those on the line in the bloody front West of Kiev are the bulk of their airborne troops, the VDV. Also on the line elsewhere are more VDV and their Naval Infantry.
There is lots of info in open source that doesn’t get into the MSM news except inadvertently. You can be your own open source intelligence analyst.
They do throw things away. Eventually. More often they just stash it somewhere where it rots away. Some years ago some photographers did a series on what had happened to all that Soviet equipment in the new Russia. And they found it, acres and acres of mouldering jets and rusting T-72’s.
A lot of that is, I think, what gets reported as being in inventory, as in those “12,000” tanks in reserve. How many of those can still move? A tiny fraction I guess.