Suppose someone switches from Windows to Ubuntu, and then Ubuntu goes full adware and becomes awful. Switching from Ubuntu to a different distro will be a hundred times easier than moving away from Windows was - all their software will work, config files for the apps they use will work etc.

That's the beauty of GPL, let a thousand distros bloom, yes it's chaotic but no one can ever really force a bad distro down your throat (except your corporate IT department I guess)