Having different compositors at the level they're at allows a lot more interesting innovations to be made, and aside from NVIDIA's tomfoolery, it has not caused significant inconsistency. Everyone uses libinput, almost everyone uses colord, and everyone implements GBM.
Wayland is substantially different from X, and the ways in which it is different are good. NVIDIA did the one thing we needed people not to do.
To be honest though, NVIDIA engineers do not spend any time engineering graphics stacks at all; they write graphics drivers for stacks that are already largely specified. They're really good at shipping hundreds of megabytes of dirty hack code to patch popular shaders from ISVs to run slightly faster on their hardware, and probably great at writing shader compilers and threaded GL drivers.
If I wrote an X compositor that supported NVIDIA's proprietary interface, it would work with pretty much every window manager that's ever been written.
If I patch wlroots to support NVIDIA, I'll still only be able to use window managers based on wlroots; I won't be able to use ones that spun the Wheel of Incompatible Wayland Compositor Libraries and landed on Weston, swc, Orbment, or whatever GNOME uses instead.