The comments threads have always been an internet Mos Eisley cess-pit, so I'm unconvinced the social aspect have ever been particularly prevalent, and the competition amongst content creators is obviously intense, but to say nobody goes there is just plain wrong.
Heck, the content glut means the likely hood of hitting relevant content quickly is higher than ever, quality notwithstanding.
> Somebody should do a video startup that values free expression and adult topics of discussion, and eat their lunch.
If you can find a way to get the people either providing or consuming the media to pay, that might work (though not so much at eating YouTube's lunch as serving a different market).
If you want to monetize that with advertising, good luck: YouTube's content restrictions follow the general trend of big advertising-supported media, because its what the paying customers for that want.
If by "dying" you mean "growing up into mass-market, big-money commercial media" then, sure, its been dying.