Disagree on that count. 7 man cons are incredibly potent infantry when parked behind green cover. I'd also point out that Soviet infantry also includes Guards and Shocks that can easily compete with their respective counterparts.
The IS-2 comparison isn't quite accurate because the Elefant has direct counters, and only shuts down frontal armored assaults. By contrast, the IS-2 will deal with everything that's on the battlefield, and it's only reliable counter is a Tiger.
Soviets have no ai upgrade outside the 7th man or nade non doc. The t70 being strong but coming latest compensates that. Shocks and guards being potent have nothing to do with cons being potent late game, they are doctrinal.
While guards and shocks can compete against doc and non doc inf, cons coudnt against almost every inf beat them handsdown without 7th man after ai upgrades hit the field. They where just a expensive snare unit nothing more. Now they can defend and force more input to dislodge them.
The tiger also counters everything sov. So why is only the is2 the problem? That said i do feel heavies in general come to early. They need a delay across the board.