Competition for non-monetary resources is absolutely a thing. Developer time is scarce and other projects can absolutely see others as competitors in this regard. We have plenty of stories of project forks sprouting because of frustration/disagreement/etc and the new fork starts gathering more attention/contributions because of better governance, better devx, saner environment, etc.

Yes, but this is not a case of project hard fork, not even a soft fork. They are two completely unrelated projects. People contributing to KDE would probably not contribute to Gnome for a variety of reasons - and vice versa - and it's perfectly fine. One aspect of open source is biodiversity.