X11 as a protocol will probably continue to work ~forever.
X11 as a display server will continue to work ~forever as long as someone maintains a display server that targets Linux.
KDE and GNOME will not support X11 forever because it's too much work. Wayland promises to improve on many important desktop use cases where X.org continues to struggle and where the design of X11 has proven generally difficult to improve. The desktop systems targeting Linux want these improvements.
> "Because it's modern" and "Because it's outdated" just aren't compelling reasons for anyone besides software developers.
I can do you one better: that's also not really compelling to software developers either most of the time. I beg you to prove that the KDE developers pushed Wayland hard because they badly wanted to have to greatly refactor the aging and technical debt heavy KWin codebase, just for the hell of it. Absolutely not.
The Wayland switchover that is currently ongoing is entirely focused on end users, but it's focused on things they were never able to do well in X11, and it shows. This is the very reason why Wayland compositors did new things better before they handled old use cases at parity. The focus was on shortcomings of X11 based desktops.
> And "because we're going to drop support so you have to switch eventually" is an attitude I'd expect out of Apple, not Linux distributions.
Yeah. Except Apple is one of the five largest companies in the United States and GNOME and KDE are software lemonade stands. I bet if they could they would love to handle this switchover in a way that puts no stress on anyone, but as it is today it's literally not feasible to even find the problems that need to be solved without real users actually jumping on the system.
This isn't a thing where people are forcing you to switch to something you don't want under threat of violence. This is a thing where the desktop developers desperately want to move forward on issues, they collectively picked a way forward, and there is simply no bandwidth (or really, outside of people complaining online, actual interest) for indefinitely maintaining their now-legacy X11-based desktop sessions.
It actually would have been totally possible, with sufficient engineering, to go and improve things to make it maintainable longer term and to try to backport some more improvements from the Wayland world into X11; it in fact seems like some interested people are experimenting with these ideas now. On the other hand though, at this point it's mostly wishful thinking, and the only surefire thing is that Wayland is shipping across all form factors. This is no longer speculative, at this point.
If you really want to run X.org specifically, that will probably continue to work for a decently long time, but you can't force the entire ecosystem to all also choose to continue to support X.org anymore than anyone can force you to switch to Wayland.