Anything less would be inconsistent. Every vehicle with the ability to counter ATGs head-on has been nerfed fairly severely: Tiger, KT, Brummbar, Scott (sort of, with the scatter changes), Pershing, etc.
What if... now sit tight.... its MEANT to counter ATGs... because you know.... it outranges them?
Its a unit designed to wreck static defence from long range.
Even in the current balance preview patch, the OKW Tiger lost its "Panzer Commander" upgrade for this exact reason.
Outside of being limited to 1, these units have nothing in common, so using one as example of what should happen to other is void point.
Leaving the ISU-152 as the sole exception, while also having it perform much, much better than any of the previously nerfed units doesn't make sense.
The unit -IS- a sole exception.
Like many other unique units.
All the other units are either stock, much wider available, can't be killed by 1 tank or 1 AT infantry squad getting next/behind them or much more mobile, often a combination of all of the above.