Yeah, I've seen libdecor as a solution, but it still feels quite off to have pretty much every wayland client have a whole dependency for such a trivial thing.
Yes, the client is supposed to manage the client content, but the obvious question then is whether the window decorations are part of its content. In some cases (stuff merged into the decorations) it can definitely be the case, but, for most things I'd say the decorations are as much a part of the client content as the apps entry in the taskbar (both contain the title of the app, potentially the icon, options to close/maximize/minimize). The only difference is that decorations always appear immediately above a window, but even that isn't really a fundamental part.
As someone who has written client code targetting X11, it's indeed quite unfortunate that, to properly target Wayland, it'd need to all be replaced, but... good riddance. Working with X11 was fucking hell. X11 has so much broken/unreasonable garbage in, like, most places. Working with X11 has been, by far, my programming worst experience.
This is not to say that Wayland is automatically better at everything (I haven't looked into it much, and the server-side decoration problem is indeed a problem) but it'd be damn hard to be worse than X11 or be anywhere close to it.