@Freddy Your claim it a bit funny, to be honest :) You say that we don't live in 80s any more, the world developed etc. But then so the hardware, there are 32 and even 64 bit *fast* CPUs running in machines with Gbytes of memory etc etc. So then if your claim is true, then the core problem already: why someone would bother with a 8bit system basically, if world has evolved a lot since then? Yes, because the desire for that technology, ie "retro computing". But then, the same way, as hw evolved a lot and you can use a much faster computer now (like a modern PC), it's true for OS designs/UIs as well, so for the retro feeling (if someone wants that, and by choosing a 8 bit system nowadays means that!) implies the feeling of a 8 bit system (the reason why you still care, regardless the 99.99% of people who moved further already ...). Surely, you can do a very fast 8 bit system even near the GHz scale with Gbytes of memory and you can run a clone of Windows (being similar in look&feel) on that, but then what's the point and joy there? Nothing offensive here from to argue, just I can't get the point ... Using even a modern recreation of some 8 bit system shouldn't forget (too much!) the original goals/feelings etc attached to, or otherwise it becomes a pointless effort. In my opinion ...
And surely you can use any OS you want, no problem, just making literally "default" sounds odd for me. But maybe a minute/moment or so it would take for the user to do that, and if somebody wants they can make default for them. I cannot see the problem, but I don't know why it would be so important to force "being default" something, which is totally alien for the "image of the platform" itself.
Again, personal opinion only.
For C900, AFAIK getting Amiga made that efforts obsolete. In my opinion Commodore often has the problem that "they didn't know what to do". For the 900 they seems to be more focused on the business side, by contrast, the targeted audience of Amiga is not exactly the same, or was not clear. Still, later Sun Microsystems literally "prayed" CBM to join efforts for Amiga+UNIX+business sector, but CBM didn't go that way with them (who knows, otherwise there would have been Commodore branded servers, workstations etc in later as normal thing ...). The mistakes of Commodore plus 4, which was the reason of failure (Tramiel didn't want that for another C64, but after his leave, others tried to sell it as "another C64", it was not the plan, the failure is quite natural here). Or just see Commodore LCD, which would have been a very advanced mobile "business class" stuff in its time. Then well again C65, which has been started to be developed WAAAAAAY too late, I am not even sure why ... Commodore should have been focused on developments on their modern flagship products, ie Amiga line. Surely only on "strict" technical/marketing sense. No need to say, I love even new 8 bit computers, even today, but it's more like a hobby, personal enjoyment etc rather than the bleeding edge of our technology as the humanity, of course.
C128 is again an interesting thing. According to Bil Herd, even the C64 compatibility was not the very original plan, neither including the Z80. And the painfully slow Z80 clock speed was more like a hardware level development problem that they haven't got enough time for a late idea from somebody to drop in a Z80 too. Or at least this is what I remember about this topic when I discussed this with Bil.