Waylands design is even more broken and leads to common functionality being duplicated and broken all over the place. Each app has to do input handling on its own? Comeon, a 5yearold could tell you that that is a huge design flaw. There are equally problematic design flaws in X11, just less of them and in different areas. Where Wayland tries to get rendering right and botches all the rest, X11 is weird for rendering, but at least has kind-of-ok answers for remoting, input, clipboard, screenshots, etc.

There is no reason why window managers can't share functionality via libraries. In fact, some do (wlroots).

X11 and its separation of graphic server and window manager encouraged code reuse by placing it in the server, but with Wayland that separation (and the extra context switches) are gone so there is less incentive to share code.

Which would be fine if we had something like COM to enable portable interfacing between libraries and certain level of separation, especially when a library plonks something breaking your runtime by messing with global resources (for example, threads and signals).

But we don't, and the libraries push other issues into your design as you often are forced to follow their specific idiosyncracies.