I am usually on the pro-Wayland side but with GNOME and KDE the Wayland implementations are fairly independent. That means that your statement that KDE going “Wayland by default is going to benefit gnome too since it’ll put more priority on bugs” is watered down somewhat.

Fixing bugs in the KDE compositor / display server ( KWin ) will not necessarily address bugs or missing functionality in GNOME ( Mutter ). A lot of what they share is also shared with Xorg ( libinput, libdrm, KMS, Mesa ).

On the application side, apps lean heavily on the toolkit libraries. KDE apps are built with Qt and GNOME apps are built with GTK. Fixing Qt bugs may not improve the quality of GTK and vice versa.

Smaller projects will share more infrastructure. Many other environments are using Wlroots as a compositor library for example. Fixing bugs there will benefit them all but again is independent of KDE and GNOME.

Your point is still valid though. For one thing, the larger the Wayland user base, the greater the number of use cases the Wayland protocol itself will be adapted to address and the more testing and development everything in the Wayland ecosystem will get.

Over time, one benefit of multiple implementations will probably be code quality. Apps that run well in multiple environments are well implemented and the same is true of environments that provide the necessarily features to a large body of apps. In that way, more bugs will be found and fixed in all environments.