X.Org vs. XMir On KDE, Xfce, Unity Desktops

[mrugiero]

Wayland allows extensions, yes, but that actually only prevents fragmentation - it ensures that no company will need to fork the entire protocol to suit their needs, they can just use extensions. And it's very hypocritical of Mark to complain about "extensions hurting compatibility" when Canonical has no intention of maintaining any kind of compatibility for Mir...

I don't think his argument was about compatibility. Of course, compatibility wise there is no way to misunderstand the fact creating a whole different solution will almost surely break compatibility more than using extensions. I think he says the extensions will lead to a system that gets in the way and such. If that's what he meant, well, the fact the core protocol keeps lean makes for better modularity. X's problem wasn't that it allowed for extensions, but that it included in the core protocol way too many things, which might have been something good back in the day, but is an obsolete and bad approach for today (specially because then you have to support all of this cruft because of legacy, you can not break compatibility with X protocol and still be X).
Mir is actually going the X way, if they are against extensions, since every change will end up in a fatty Mir server. I don't think they'll handle it that way in the end, but Mark's complain makes it sound like that.