Unless Axis heavy TDs like Ele and Jagdtiger are going to get their ranges reduced, ISU shouldn't be touched OR should be compensated to be more mobile. Is the ISU's HE broken? Yeah, I'd say so. But it's such a crutch that making it more vulnerable will put late game soviets in an incredibly precarious position without some compensation from the axis side.
Have you played any games on the patch mod? I feel that you're overestimating the impact of the proposed changes.
Fact of the matter is, the ISU, as you said, is a crutch unit. That is not a good thing. It is far too easy to put to devastating use and inversely far too difficult to counter. The ISU should be more vulnerable, it should not have things shuffled around so that it remains as such a no-brainer counter anything unit with little to no downsides.
Axis heavy TDs already have big vulnerabilities. Hell, both ISU commanders come with IL2 bombs that make short work of them.