Most distros have this no bundled libraries rule, so either the stale version can still be used after an update, or if it's yanked out from under running proces

Most distros have this no bundled libraries rule, so either the stale version can still be used after an update, or if it's yanked out from under running processes, they can crash. Hence rebooting. macOS and Windows have mandatory reboots for anything system related too, so this is not out of the ordinary. Right now, if you use flatpak based applications on Fedora, they get updated without reboots. And that's the way forward, including for the desktop environment itself eventually.