I think alternatives could exist and be extremely popular, especially since people are crunched for time. On the flip side, sites like HN exist for the insights provided by comments.
HN is fantastic. It doesn't feel bulky because commenting is integrated with the original idea of the site. In fact, you can't do anything but read and leave comments.
For another site though, like a blog, or a photo sharing site, or even a news article you read online- there aren't a lot of alternative to comments.