Consistency. One OS version for all units. One code base to troubleshoot, modify and maintain. That's a huge deal and no small thing. Adding a circuit for fuzz is an extremely simple thing in comparison.
Second - the Stage is 8 years newer. I would hope it had some features the older models don't. It'd be nice...but it sure looks like Kemper has other ideas they want to explore. They didn't make this change for nothing.
In principle yes - but you cannot as a company just ignore that some of your hardware have different buttons/feature that need to continue working. Right now we have an EQ button that does nothing and 2 delay LEDs for the delay module that makes no sense. It's not the path to take. I would be extremely pissed if the STOMPS and EFFECTS buttons on the rack stopped working just because some other version of the hardware don't have them.