>[...] a modular basis for Sway and other Wayland compositors to build upon
I was curious to see what that looked like so I went on the github and the first sentence on the README is:
>Pluggable, composable, unopinionated modules for building a Wayland compositor; or about 50,000 lines of code you were going to write anyway.
My jaw literally dropped when I read this. It seemed so wild that I actually cloned the repository and ran sloccount myself to check if there was a catch (there isn't, master is at 53k lines). My DWM is 3k lines and it's fully featured as far as I'm concerned.
I realize that it just pushes a lot of that functionality (and code) into X but at least it separates the concerns, my WM doesn't ship with half of the X11 source code as a hard dependency. Also X11 has been battle tested for literally decades by now, it's not a fast moving project (well, arguably it's quite the opposite, hence the very existence of this discussion).
I haven't looked very deeply at Wayland so I won't say that they're doing it wrong, maybe I'm just missing an important aspect, but the more I learn about it the more it feels like they've thrown the baby out with the bath water.
X11 can be hugely hacky at times and some of it is seriously outdated at the conceptual level, but it also does many things amazingly well, arguably better than any other mainstream desktop environment out there. It's an incredibly flexible, if a bit idiosyncratic system. Wayland seems to fix some of its flaws by introducing a brand new system that comes with its own set of drawbacks.