This just false. There are libraries you can use to get most what you need and you can only change the things you care about related to window manager. And many systems have done that. Libraries are somehow a good way to share software, but not when it comes to display managers?

The claim about OpenSource manpower is also false. We have multiple high quality compositors and despite any issues, there is quite a large amount of homebrew and other compositors.

If you want something like X11, you can built on something like Mir.