This is all playing a bit fast and loose with the details.

The "isolating windows from each other" stuff in Xlibre for example is the Xnamespace extension, which requires a static config file up front and lets X clients within the namespace interact as before. This may have some utility for specific scenarios (dunno, kiosks maybe?), but is nothing like Wayland's default security model.

Similarly, enabling TearTree in the modesetting driver and having another backbuffer in the driver is a huge crutch vs. having a proper architecture where the compositor can own presentation timing. For one it makes adaptive sync/VRR a lot trickier.

These things are overall not equivalent.

It isn't clear why any of this would require a rewrite.