Couple of notes, it would probably be the su-125 (the t-90 has a 125mm, and if its just using the same gun and not in a turret, it may as well just get that name.) Alternatively, Soviet naming convention would make it “t-90 mod. 2025” or western influence “T-90 AT”

Either way, upon visiting tje Wikipedia for the su-100, I found many Warsaw pact countries retain some even today. Some armor is better than no armor when you can’t afford newer toys. And a 100mm gun will always be a 100mm gun to fight a rebellion/invaders handguns and rifles.

They could have made something great by making something more like a Stridsvagn 103, but they are Russians, so they won’t.