The first thing i thought of when i saw the feature is adults. Kids need to learn what clickbait is; turning it off wont help them with that. Plus kids learn thus stuff much quicker than adults. Adults understand clickbait, but its at times an eyesore and can get tiring to deal with. Is it only available on family plans for money reasons? Maybe add it as a + tier option for non families or a paid add-on?
I do agree that it shouldnt be turned on by default, not just for the reasons listed, but because some channels are really good at thumbnails. I like awarding youtubers for good thumbnails (plus it helps me find content) and punishing youtubers for bad. If this silently went live, id probably not notice for a long time, and once i would have, id be pissed.