You inspired me to write a longer story on how I see an alternative history of computing without the PC: https://erik-engheim.medium.com/a-world-without-the-ibm-pc-9db63c27b85e
It doesn't answer all the points you made. I think I still struggle to see how you can call the x86-64 streamlined.
It still has complex address modes. Still has 1-15 byte long instructions. Still has this weird hotdgepodge of registers. As far as I understand MMX, SSE and AVX are all kind of overlapping. So things seems messier rather than streamlined.
The segment registers today just come across as odd vestiges of an old architecture. They serve no real purpose.
So no, don't try to tell me x86-64 is anything like the beautiful M68K. Just some googling to figure out the current state of x86-64 left we so confused, there is no way that thing is streamlined. M68K was something you could pick up really quickly. Not so with x86-64. But I guess it is all in the eye of the beholder ;-)
Sure PCIe, USB etc are more advance. But I don't see how it logically follows that similar advance standards would not have developed without the PC. Quite the contrary I think we would have gotten there much sooner. So much PC engineering talent went into working around oddball design decisions in the PC. I mean the PC was pretty much made as a stop gap measure. 8086 was just made by Intel to stop Z80 while they made their "real" chip.
I do actually regret the transition of Acorn to ARM. I think a world dominated by Acorn computers would look a lot nicer than a world dominated by PCs. It is not like ARM CPUs could not have ended up in mobile devices if Acorn was still around. If not ARM, possibly MIPS. I don't think ARM is that special.
A world with Acorn, NeXT, Sun, SGI, Apple, Atari, Amiga etc would have been much cooler than the overly homogenous computing world we have today.