this is a topic i’ve been heavily involved with because i still consider myself to be someone who prefers using technology at a very beginner friendly level, plus it’s very good when a linux operating system makes you feel right at home when it has a modern desktop environment. this is why i really like gnome, its simplicity and usability is something available for everyone, for beginners and for a lot of other people, but if you had to, say, rearrange xfce or kde for someone who was an elderly person or an absolute beginner so that they wouldn’t have any trouble using linux, how would you do it? (screenshot is my current linux mint desktop, very simple and extremely user friendly!!!)
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.