I guess if screen tearing is the worst thing you know, you wake up in cold sweat every night screaming its name, I guess you should really use Wayland. But for other people?

Right, I never got this. Every time the X11 vs. Wayland thing comes up, I see so many people talking about tear-free rendering as if it were on the same footing as curing cancer.

(Regardless, I haven't seen screen tearing on X11 in... close to a decade? More? I don't care if that's the case only because it's "possible", and not because it's "inevitable". Who cares? It works fine.)

Have you ever seen a film or a TV show that had screen tearing in it? I haven't, it would look awful. For anything that needs to animate smoothly at a stable frame rate, tearing is a bug. It makes it look really choppy. You can do various things to eliminate a lot of tearing in X (like using an X compositor, and only using clients that implement double buffer vsync), but the default in X is that clients can send drawing commands at any time. Wayland changes this and makes it so you're practically guaranteed to never see tearing by default.

I've used X11 for like 20 years and done all sorts of things, including watching video and even doing video editing, and this has never been an issue I've so much as thought about.

I don't know how that could be possible, I find tearing to be incredibly noticeable when doing video editing. If you're getting it when trying to do transfers between frame rates it will potentially screw things up, because you're not able to see the actual frame rate. Occasionally I still see a youtube video where someone recorded a screencast and was getting tearing, and I find it really jarring.

Dunno, maybe it's just not something I'm noticing at all. Some people seem incredibly bothered by it, but I just barely notice it if I'm not actively looking for it.

I'm far more annoyed by the added input delay and sluggishness from vsync. I want frames on the screen as fast as possible.

I don't know how you can't notice it when doing video editing, you have to actively notice the frame rate to do it right. Any choppiness will be evident. Also I really don't understand your second paragraph. You can't get rid of input delay or make frames appear faster by disabling vsync. When you mux the video you target a specific frame rate, the common formats are almost always 24, 25, 30, 60, or multiples of those. The frames are appearing the same, it's whatever is in the video file.

Any form of sane security. X11 can’t be made secure unless you go full nested X servers, where you are back to square 1 (cut holes for screen sharing and the like).