We don't need it to have comments. But I think the lack of commenting is the big problem of an RSS focused ecosystem.
Shooting comments off into the ether is fine by clicking the page and commenting there. But, if you have a couple of dozen blogs in your RSS feed, and leave a handful of comments a day, clicking back through the specific place to see if you've been responded to is way too much effort. Sites like HN, Reddit, and Facebook, have a central mechanism by which you can be notified.
In the '10s I had a blog with over 1000 readers, I had fewer comments, and less community than the replacement Facebook group has with a fraction of that number.
That may not be a problem for publishers who are purely interested in eyeballs. But that isn't all of them. If the aim is to free content from the walled garden (which I think is an important goal), then it matters.
Don't most blogging platforms have the option to email replies? In any case, I prefer a bit of friction in the comment process, since it discourages people from "shooting off" without thinking.
Usenet with dial-up was good for this. You'd compose your reply, then look it back up, read over, amend, perhaps delete or send later. It made for more considered responses.
I've noticed with HN it's very swarm-like and transient these days. Comment following is also difficult without some thread control. At a minimum you need collapsible threads. Too much to read, don't bother, say something that's been said already - and add to the noise. Or have a bypassed dangling leaf comment.
Any decent commenting system lets you receive reply notifications. If some of them don't, the solution is to fix that, not split the userbase by constructing an entirely new, parallel comment system.
I've seen email notifications, but to me they are just as bad as subscribing to a lot of sites by email. That's kind of what RSS is for: aggregation. So yes, it means you don't have to click through sites, I accept that, but still not a good user experience. If you mean something else, let me know, I am interested.
I don't know what you mean by splitting the user base or constructing a parallel comment system. I'm just talking about how to provide commenting, as a content publisher. RSS + on-site page by page commenting (even with email notifications) has a fraction of the engagement (in my anecdotal experience) of FB.
