This is not something conceptual about Wayland, or intrinsic to it. Any display protocol can have multiple client/server implementations. It just so happens that most of the free desktop standardized around the X.org implementation for X11, but even for X11 there were multiple client and server implementations.
The second thing is that software duplication efforts have a well known, tried and tested solution: implement the duplicated effort in shared implementations or libraries [1] !
The thing I find hard to wrap my head around is that this complaint is given in a community that usually champions protocol-over-implementations and diversity of implementations. The people actually making a difference on the Linux desktop are damned if they do and they're damned if they don't.
What you want at that point is to agree where to read and write common configuration. I'm not sure this is likely to happen either, as gnome has gsettings/dconf and kde has simple files and you will now awaken the plaintext vs efficiency dragon.
The current situation is just how it coincidentally turned out - there's just not always a clear direction on the free desktop (and that's even what many people like about it). If wlroots or something similar had existed earlier, then this probably would have turned out differently but c'est la vie.
edit: looks like it might already be provided via wl_output::scale [1] except you get the wrong effect that way. You want to tell the client what DPI they should render at, so that you can keep scaling at = 1
[1]: https://wayland.freedesktop.org/docs/html/apa.html#protocol-...
Once that ship has sailed, what you want instead is a shared method of providing configuration. The goal is to have all 3 implementations read configuration (like DPI scaling etc) from one place. That way the user won't have to configure common things in 3 places to get any semblance of apps looking remotely close to eachother.
When people talk about "pushing stuff to the DE" what they mean is the Wayland project aggressively scoping the Wayland protocol to drawing surfaces and handling input events. They're not trying to become a generic desktop message bus for everything an application might want to do -- that's literally what dbus is.
Why not have a shared implementation?
The codebase could be re-engineered, sure, but a lot of people championing Wayland tend to see this duplication of effort as good for some reason, and at least last I engaged with them did not like the idea of a "new X11."
Personally I think something like that is an inevitability.
People seem to like that we have many browser implementations but when it comes to display servers it seems like they want Chrome to rule them all.
Diversity is great but equality in failure is not helpful.
X11 is a protocol. What you mean is that we end up with something like Xorg, an implementation of X11. Which is still incorrect because a major design goal of Wayland is to move as much logic as possible from the server to the client.
Wayland is bad design, and it shows in the users' and maintainers' attrition.