Are we really pretending multi-monitor on X is not an absolutely shit show? Try mixed DPI and VRR in one of the monitors and see how you get that working well.
Now drag a window from one monitor to the other. The choice on X is to have both monitors run with the same specs (well, not for everything but yeah) or to have two completely separate screens at the X level which means they're isolated from each other and you can't move thing between them.
Thank you! X just plain doesn't work for that usecase, you'll either get blurry text on one of the monitors or a tiny window with mixed DPI. It fundamentally doesn't work.
Reading some of this thread has made me feel like I'm going insane. I have a wildly different experience of both X and Wayland from a lot of people in this thread it seems.
The VRR issue can be solved with a simple patch that never got merged into mainline (no idea why). Hence it's not inherently a X11 problem and honestly also a rather niche problem compared to the humongous deficits Wayland brings.
Mixed DPI works perfectly fine on X11 if the toolkit supports it. The xrandr protocol provides all the necessary dpi information. GNOME simply chose not to support it on X11.