X11 is not dying because of Wayland, it's dying because it became an unmaintainable code mountain over the years since it has been tried to be converted to something which is not designed for in the first place.
That's Xorg. X11 is dying because Xorg is dying and Xorg is dying because all its developers quit and went to Wayland. There's Xlibre too but it's a shitshow. Any one of us here could continue Xorg maintenance but we're not.
I think Phoenix[0] is a promising project: it's an X11 server written from the ground up, with security and legacy feature removal in mind. It's basically what Wayland should have been IMO. We didn't need a new protocol, at most we needed a new implementation of X11.
...and I'm saying that as a person who likes X11.