Sure you can do everything with a library, and that's how you end up with library hell, or dependencies that will be hard to get rid of or replaced later, because how big they become or how broken the API was (e.g. fontconfig). But being the only ecosystem where that's needed is pushing some heavy boilerplate on developers. Why would games, or non-gnome apps care about decoration? There's plenty of good examples of applications where the decoration being forced on them doesn't make sense. The protocol is also standardized and gnome is the only wayland compositor that refuses to implement it, this is not the only case where gnome devs just decide to be jerks.

Things like these are why I think I'll never will be using wayland as it just breaks everyone's existing applications and workflows. wlroots and sway developers are doing good work to try to bring wayland and its tools to same level as X11, but then we have these other groups that just actively reject anything that doesn't go along with their "vision". And I speak as someone who wrote one of the first wayland compositors, and one of the first wayland compositor libraries, which code partially still exists in wlroots.