Sorry, but the inability to hit moving targets is simply not a fun change. Reducing the usability of a unit is not the way to balance it. It simply makes the game less fun and overall irritating.
It's very clear that this kind of change will simply make these units never used.
It comes down to the fact that this ISU is nothing like the previous one.
This one needs micro and attention
This one isn't the way to go in 80% of the games
This one isn't better than going for t4 and get su85's or other things
This one isn't the beast it used to be.
The fact that the unit changes and gets a an other role in the game, is in fact also a part of balancing. Its not that easy to use.
Its not always about changing AoE, range or splash, its also about other solutions that reduce the impact of a certain unit.