absolutely zero value to YouTube
this is not true. non-paying users will like, comment, share to others making the platform more popular. and if they're ads were less intrusive they would actually be more likely to be shown.
I'd argue they risk losing users who post links to their videos online. It's likely the same group who tend to use ad blockers are also the ones more likely to link a video online, so YouTube would miss out on all the non-adblock users clicking links and watching the videos that way.
Not really. YouTube provides a lot of tools to make most of their services and content pretty accessible from third party clients. If Google wanted to prevent ad blockers and force you to watch ads, trust me they could. At this point I think some users that only know how to install third party software are going to start claiming they won and ruin a good thing for everyone. You don't want ads? Then don't bite the hand that feeds you.