We get a decent amount of comments on blog posts right here on CSS-Tricks (thanks!), but I'd also say the hay day for that is over. These days, if someone writes some sort of reaction to a blog post, it could be on their own blog, or more likely, on some social media site. It makes sense. That's their home base and it's more useful to them to keep their words there.
It's a shame, though. This fragmented conversation is slightly more useful for each individual person, it's less useful as a whole. There is no canonical conversation thread. That's what Webmentions are all about, an official spec! In a sense, they allow the conversation to be dispursed but brought all together in a canonical conversation thread on the main post.
Webmentions don't need to be an alternative to comments, although when you pop over to real Drew McLellan's post you'll see he's using them that way. They can be in addition to "regular" comments. Surely the idea of turning off regular comments is appealing from a community perspective (less asshats likely when you need to link to your published words) and a technical debt perspective.