Very interesting…
Modules… Yeah why not, but I bet some things have to be change very deeply before that.
Bit it is almost certain that everyone should find what (s)he’s looking for, with the benefit of the flexibility…
Not for tomorrow exactly 
Now, about drawing the line, I totally agree, it’s very difficult, if not impossible.
Especially because on occasion post guys have to deal with music… Well in the music production way, sort of, and vice versa…
In my opinion at least Steinberg let us cross grade from C to N. Good. But surely not enough in a perfect world. An ok to not talk about NEK.
Specific post stuff ?
ADR, edl, conformation & bwf field recorder
Metadata management, even way more aaf (and hopefully soon IMF) filtering, some advance video management (not sure about that one, I think a lot of music guys need it as well)
Don’t shoot but I think automation as it exist today in Nuendo is good, but still not enough for post (maybe enough for music ? At least of course if you use it as intended and certainly not as PT automation, which Is really great on a lot of points, but really different in essence)
Advanced Surround feature like Iosono Anymix sounds definitely post essentially (including scoring, but not music-only, yes?)
But the opposite is also true.
For instance, 4 cue sends is more than enough in a lot of of post configurations, but I totally get that it’s not enough for music…
And about the price I personally would be ready to spend 50% more on a Nuendo license with more post features and the ram pressure break points problems solved.
Nuendo is not so pricey imho… Plugins are.
So I don’t really get why the update fee is so problematic.
At the same time, we have to pay for new functions, and the annoying and/or critical bugs are still there… Now that’s problematic…
Like many, I’d rather see an upgrade (even with fee, yes) solving issues than new (really cool, yet) features.
For the solo/mute problem, I get, but it’s also a POV problem :
In front of a console or a controller, you certainly not solo/mute with a keyboard… Problem solved.
When you edit, you mostly use the editor, and the mixer way less. No problem either in that case.
But I would agree that especially with the mixer only in front of you with no controller at all, it’s problematic.
Simply does not concern everybody. Maybe the mixer functions where designed with a little too much of nuage in mind…