If YouTube was just looking out for big brands and/or advertisers, they could have restricted the change incredibly simply. By making it a global change they're saying "ok now we understand how our UI actually led to some pretty bad outcomes in ways we failed to anticipate, and we're fixing that".
* clickbait headline videos that don't deliver - ie, content in thumbnail never shows
* obviously some political content has been getting dislikes which is embarrassing and risks govt coming after them. Trump threatened this and dems are also now making noise here, so these platforms are vulnerable.
* Dislikes were pretty rare, usually in my case I had to be really annoyed at something to dislike it. I didn't mind scams that were properly described, but some stuff is annoying because it's a bait and switch.