What you would want to mix is a window manager, and there is no reason why that can’t be done with this model (and is pretty much what wlroots does).
But I do agree that a cathedral model can have benefits.
With X11, if someone wanted to write a window manager, they could do that and only that, without having to write a compositor or display server. Wlroots certainly simplifies those latter two bits, but there's still a lot of work to do to use it.
Beyond that, Wayland's mess of extension protocols are generally incomplete. I've been working on "porting" parts of Xfce to Wayland, and there are a significant number of things that just cannot be done at all right now without integrating it directly into the compositor process. The extension protocols necessary to do even a fraction of this table-stakes stuff are often experimental or non-standard, and even when they do exist, are often missing critical pieces.
As an example: say you want to write a dock-type app that only shows the list of toplevel windows present on the current workspace (and when you switch workspaces, it'll show the windows on the new workspace). Nope, you can't do it. The workspace protocol itself has been languishing in Freedesktop GitLab[0] for two and a half years, and even if that was standardized, it doesn't interact with the Foreign Toplevel protocol at all, so you have no idea what windows are on which workspaces.
So currently the only solution is: your dock app has to live in the same process as the compositor, where it has access to all the compositor-specific data around workspaces and windows. (Sure, you could build a d-bus interface, or something like that, but that would be specific to your one compositor... and a d-bus interface is the moral equivalent of a Wayland protocol, which is what you should be using anyway.)
[0] https://gitlab.freedesktop.org/wayland/wayland-protocols/-/m...
Er, are we using different meanings? Obviously something has to display pixels, but I literally do run X without a compositor.
> a window manager, and there is no reason why that can’t be done with this model (and is pretty much what wlroots does).
No, you can't (excepting rootfull xwayland, obviously), Wayland forces the display server, compositor, and window manager to be a single program. All wlroots does is make it easier to write that compositor, it hardly changes the model.
