There isn't much difference there, but it would be more of an uphill battle if you tried to put everything different that Wayland does into X extensions. That still requires maintaining an extra code path for old X servers that don't support the new extensions, and creates additional risk of breaking things and causing regressions in the X server because of all the new code you're adding.

Clients like term, xpdf, and xfig should work fine in XWayland. Window managers won't work without getting ported, but someone has been working on a port of Openbox: https://github.com/johanmalm/labwc