I found Xorg crashyness depends on how shitty your drivers are. Agreed that merging the components is a bad idea. Under KDE, there is more of a split, kwin does compositing and WM stuff, and plasmashell does the shell and other UI bits. In theory they don't need to all be in one process either. I also found that kwin crashing doesn't bring down apps these days, haven't tested that theory with X11 Xwayland apps though. Also see my other comment about Arcan for proper crash resilience.