If that day comes, I'll might celebrate, because YouTube comments are mostly worthless, full of sexism, racism, insults, and sheer idiocy. I rarely come across a YouTube comment that imparted useful information.
G+ doesn't really need propping up, most of the people I enjoy reading are on it, I get more engagement than I ever did on Twitter or FB with the people I'm interested in (note, not friends, but interests), and with a less infuriating comment system. I'd personally rather not have G+ turn into FB.
In fact, I take back my first sentence, now that I think about it, I'm somewhat fearful of YouTube adopting G+ comments, because rather than improving the civility and intelligence of YouTube's comment audience, it might destroy the civility and intelligence of the G+ community.