I have no problem with people disagreeing. It's the bitching and name calling that ticks me off. If their vision does not fit yours (people in general, not you in particular) then switch to something else. I was a hardcore Ubuntu-user for quite a few years. But since I felt they were going in the wrong direction, I switched to Fedora a few releases ago. And that was the end of that.Anyway for me, and it seems many others, it's more the "GNOME brand" and "shaping the user experience"... kind of thing they keep spouting. I'm sure for some this seems like a great vision but it doesn't feel right. To me. But hey, that's why I use KDE nowadays. I'm just giving my views.
GS/Mutter have been very stable to me, so I have not seen that error message. I am just saying that I don't see it as a problem that software builds on other software. All software on my machine is tied to some other piece of software. If anything lower in the chain falls over, everything on top of it will obviously do too. GS sitting on top of Mutter is really no different than all environments sitting on top of X, which sits on top of the kernel, and so on.But why? For example in KDE, if Plasma desktop crashes, it restarts itself and everything else is still running just fine. So, minor annoyance to the user, but everything still intact.
Any time something goes boom in Gnome-Shell, in my experience, after a error screen that is as useful as most Windows error messages (as in, not very) you end up back at the login manager and have to start again.
Also I actually don't mind Unity when its working well. But it's a concern to me why someone would build a desktop into a, at times, fragile accelerated environment when instead you could build the desktop itself and then add the compositing integration.
Modularity is of course nice when it makes sense. If it is trivial to make something modular, then go ahead. Often, it is not, so unless you have good reasons to make your software modular, the problems are likely to be bigger than the benefits. Since I can think of zero benefits to GNOME to have GS running on anything other than Mutter, I can see why they did as they did.