That "X" would basically be what's running on the ARM surface and arguably the whole line of precursors back to CE could be described like that. They end up wildly unpopular because they have all the bells and whistles people begrudgingly accept to get the one feature they want (continuity) but lack this one that they accept the others for. Continuity isn't just for enterprise. Non-technical users abhor having to re-learn anything or give up the procedures they struggled so hard with adopting (there are probably still some who feel uneasy if they haven't run defrag for more than two months, despite of having switched to NTFS more than a decade ago) and gamers and the like have lots of niche hardware and software solutions that easily puts the unsupportedness of vintage enterprise software to shame. Continuity is a key feature.

What I think could work, and would eventually lead to the same situation as "Enterprise/X" would be the reverse: a "regular" Windows aiming to eventually build a Microsoft equivalent of the Apple walled garden and a "continuity subscription" that's more conservative than what Windows is now (trivial example: opt into Cortana instead of opt out, barely)

PS: I understand that you are talking about low level technological differences and I seem to be hung up with UX, but I believe that separating groups on the high level would be the only viable way to eventually follow different paths on the low level.

The worshiping of windows compatibility /continuity (which I do consider to be impressive from an engineering standpoint) is astounding to me. It leads to what we have now - which, to me at least, is a frankly unusable operating system. A hodge-podge of 20 years worth of patterns, menus and systems. Some menus in windows feel they're in a completely different operating system.

This is why they should have 'Enterprise' and 'X' to me - they dont even have to look at different, but it frees one windows kernel team to do whatever they want and not have to worry about supporting loads of legacy systems. They'll naturally deviate.

It's actually kind of ironic to me that the best feature they've released in years (WSL) is essentially jamming another operating system into the mix. Like there's not enough going on there already!

I'm reading so much about how good Proton is that I'm considering dropping windows even for my gaming machine.

I consider the wildly mixed UIs not a symptom of compatibility but a symptom of the succession of hopeless attempts to be more than that. The incomplete new GUI generations are surely not caused by compatibility burdens affecting the kernel team.

No but they're a symptom of the 'all work on the same product' situation - if they were split, the people wanting to update the UI etc would work on the new version alongside the kernel team for the new version.