All I can say is Relic's own data showed the discrepancy to be overperforming in favour of axis heavies. Now axis will have to work just as hard to destroy Allied vehicles while preserving their own.
What makes you think allied medium vehicles were any more viable against shreks and panther than viability of axis mediums that you are worried about? What you're worried about now has been the reality for allied players for a long time. Allies have to rely on vehicle based AT and it's not out of choice.
Well, the ZiS is quite good. Soviets don't really lack for AT options at all. USF needed a better AT or something and better Zooks instead of a Jackson buff (IMO).
Axis mediums either don't exist (for OKW) or are to expensive (Ostheer), the Jackson really didn't help these matters either since the only 240 damage tanks Axis had were 245 fuel+.