I think it’s a fair point that immutable distros are still a developing concept and probably not ready for primetime, but I do feel that eventually this must be the answer to make things absolutely bulletproof for people (of which there are many) who have a natural ability of nuking their OS.
Why would it have to be KDE or XFCE? GNOME as it exists today is probably the closest to macOS’ design language, and whatever you do designing any kind of operating system UI, macOS needs to be the benchmark. GNOME 2 in the olden days was my favorite DE so today I have to go for things like KDE or MATE but I’m not a novice and I can fully acknowledge that what works for me isn’t what works for everyone. GNOME probably made the right decisions to lower the entry barrier towards Linux.
XFCE to me is purely a choice for outdated hardware so unless your extreme beginner has an extremely old PC XFCE is a non-starter. Whether KDE can be made more accessible to the non-initiated I can’t say. Anything that has a Windows-esque taskbar is probably ill-equipped from the start as well. Again, macOS is as easy to use in a keyboard and mouse sense as tablets and smartphones are with our fingers. This is the way.
To me it’s like the old saying from Win9x days “Help, I’ve deleted the internet”. Removing an Internet Explorer shortcut from the desktop might be easier than nuking a Linux installation, but I think if the point is to be of service to absolute tech-illiterates then you need to be ready for anything, any edge case. And like you say, if Flatpaks and all that just work then what would be the downside of immutability?
I mean in theory one can make deep-rooted changes to macOS, but I haven’t once seen it fail to upgrade successfully. And I definitely wouldn’t consider Windows immutable with how many things can go wrong in practice. I’ve had more than a few customers who lost wifi drivers through no fault of their own, in one case the entire device wasn’t visible anymore, so reinstalling a driver wouldn’t have done anything, all I could do was roll back the feature update and ensure it won’t install it again. In that sense, immutable Linux might even be the best offering out there today. The only thing it lacks (besides Android and SteamOS) is a multi-billion dollar company backing it for wider desktop adoption.