“Just stick to F39” is not a long-term solution because it only buys us 7 months until EOL is reached. If anything, it will lead to a lot of users running an EOL Fedora for years. Or they will move to another distribution.
Is it really such a burden to keep building the X11 versions of Plasma and KWin? They are built from the same SRPMs as the Wayland ones, so it does not even require maintaining additional specfiles, unless you deliberately force them to be packaged separately by dropping support from the main SRPMs as planned in this Change.
In the end, I think the proposed Change will actually increase the overall maintenance burden in Fedora significantly: you get to drop a few lines of specfile, but someone else gets to maintain a specfile that is mostly a copy&paste of yours, just built with different CMake options, and needs to be kept in sync with your frequent Plasma updates (which you will even push to stable without waiting for the packages you do not support anymore to be updated, as recent kdepim and kf5-solid updates breaking blogilo, trojita and plasma-pk-updates until I fixed them have shown).