Things like global cursor coordinates, screen capture, and window automation are intended to or already are being done through things like xdg-desktop-portals which ask for user for permission before an application is allowed to do these things.
You CAN continue to use X11. There just isn't much point for most applications especially since XWayland lets you run X11 applications in a Wayland session. Wayland is more efficient than X11. Phoronix recently did benchmarks with games in both and Gnome's and KDE's Wayland sessions outperformed the Xorg session while using less power even though those applications were using XWayland.
"Why not just integrate Weston X11 backend into Xorg and then let application developers to drop support for X11 completely, while letting me keep using X11?"
That isn't how any of this works. Weston's X11 backend, just like the X11 backend of all DEs, relies on Xorg. There's nothing worthwhile from a Weston's X11 backend to implement into Xorg.
"Do we need an IPv4 vs IPv6 battle again?"
I mean the same people who worked on Xorg work on Wayland.They didn't come up with it for no reason. X is a protocol from 1983 and has been on version 11 since 1987. It predates GPUs, OpenGL, compositors, and multi-core desktop CPUs. They actually explain the rationale behind Wayland and what's wrong with X11 in the Wayland FAQ.
https://wayland.freedesktop.org/faq.html
If you look at the ideas that were drafted for X12, you can see that Wayland is pretty in line with it. https://www.x.org/wiki/Development/X12/?action=diff
"Moreover, those «program X uses Wayland, but bla-bla-bla... set this environment variable to use X11 backend» are frustrating."
I don't know how else you would like that work.