You're mixing up BTR and Tank here, and they play different roles in combat. If you don't have any tanks left in the batallion, you will probably use a BTR. But when you don't have any BTR you'll use a IFV and when they run out of those you go on a motorcycle or in a civilian car like a lada, or by foot.

That's what is already happening.

Also I see not much Explosive Reactive Armour on those tanks from the sixties lol.

Running out is like a bag of candy, you can always grab more, until suddenly, you cannot grab anything any more.

Russia is scratching the bottom already, and will soon run out, with the exception of a handfull new ones a month, and maybe some North Korean ones lol, they already depends on them for basic shells.