Realistically, KWin will be the owner under Wayland which will be there before PowerDevil and will restore lockscreen should it restart.
Note that even KWin can be restarted (with --replace) without killing the session, and I don't think there's an explicit dependency on ksmserver to ensure order of startup. Generally watchers have turned out to be a good thing for reliability, but less complexity is also good, I trust that you'll make the right call here.
If it's broken, how much would it take to fix the watcher instead of removing it altogether?