They also use different caliber shells than the modern 120mm smoothbore of the modern tanks.
Russia has millions of the old caliber shells for the old tanks, sitting in warehouses doing nothing. T-55s use 100mm, T-62s use 115mm.
This means they can be deployed as mobile artillery behind the battle front, without being an additional draw on the newly minted from the factory 120mm shells going to the new tanks.
“Russia has millions of the old caliber shells for the old tanks”
Perhaps. But in what condition?