You reminded me that Disqus will suppress very long posts by default, which looks like this, with a “see more” expansion at the bottom of the post.
I checked around on a bunch of Disqus enabled sites and the posts have to be fairly long to trigger this length suppression, though. It also doesn’t come up very often in the “average” web page comment scenario from what I can see. For example here’s a contentious discussion about hate speech on Disqus blog itself with 267 comments total and I see … 4 very long post suppressions? 4 out of 267 is 1.5%.
One plausible solution is to suppress for length, in exactly this click-to-see-the-rest manner, posts by new users, or users below a certain trust level. We certainly would not want to do this globally since Discourse is generally about encouraging longer discussions, not suppressing them!
(There are of course max length limits on posts in Discourse but they are pretty generous!)
They push everything else down the page, and having to read through thousands of words of… stuff, even if you are just scrolling, has a real mental cost. That said, it’s absolutely effort to create multiple long replies, and a lot of effort at that. So it isn’t nearly as common as “lots of replies”.