It’s not that bad imo, probably good UX wise. People using gnome have a specific mindset/expectation that usually does not apply to people using KDE (and reverse).

Most of the heavy lifting is done by lower level libs anyway. So the duplication is not as extreme. E.g Firefox doesn’t gel well with KDE by default. And thunderbird looks quite foreign.

I always did make -j$(nproc --ignore=1) to avoid this while building cpp code. But this problem seems to be less severe if there are a lot of cores.