Universal binaries are hardly a new concept for Apple's OS. They, of all companies, know all about the...quirks...of mixing processor architectures
In the past though, whether Universal binaries were for 68k/PPC or PPC/Intel, notice the OS was always the same. You built a single code base, and compiled for multiple architectures. So logically, macOS and iOS should be nearly one and the same for this plan to be effective. And while I like that idea in principle, it makes me nervous, because Apple's infamous tendencies toward mobile-oriented experiences would hint that it is more likely for Apple to remove valuable features from the system. And/or switch Macs to ARM architecture (actually not an unreasonable or novel prediction).
Imagine having to jailbreak a Mac. That would be horrifying.
why isn't Opera there? Why not the Arturia and Korg synths? And so on)
Can't speak for Opera of course, but I wouldn't be surprised if most pro music software wouldn't make it on there regardless of reasonable changes. Soft synth manufacturers really, really, really don't want to play by anyone's rules but their own, especially the more they cost. MAS would make licensing schemes like EastWest's (using iLok) impossible, and some like Native Instruments are probably perfectly comfortable with their own licensing methods. But that's a bit of a tangent