ISU is just a bad design. It should deal zero damage to medium/heavy armour. It should only have HE shells.
If panther's gun cannot deal with infantry it is perfectly acceptable for isu to deal only with infantry. Soviets have it both ways on too many units. At squads/guns can deal with both infantry and tanks (guards, penals, ZiS, Su76, etc). If they were realistically speaking inferior at both these jobs to dedicated units (those dealing only with one threat) it would be ok. But they are too close to being as good as dedicated units (realistically speaking). This is the core of the problem. Very often some other abilities (mark target, ram, off maps, snares) combined with those units lead to problems and frustrations on the receiving players end.
Try to use your noggin here for a second. Do you REALLY want to fight an AI only unit that costs as much as the isu does?
If you remove its AT it would need a cost decrease.... Which means it would be easier to get and support. It would mean when you take one out, the next one is going to replace it eaiser.
OR
if you want to keep its cost up you need to increase its power somehow, and since it's only an AI unit that's another direction nobody wants.
Its weak AT performance keeps its cost up and its power manageable.
Balance is a teeter toter of many factors, if you simply remove one something else either has to give or something else needs improved.