First, again, by the time this rolls out (if it rolls out) there'll be a pretty prominent checkbox for "don't collapse things", so you won't be forced to view things this way (and this'll more prominent that the current user option is)

(The following is just explaining why I prefer the SingleLineComment setup, or something close to it, not trying to explain why your preferences are wrong. :P)

The frame for why I think this is important is not "I want to find information about a particular thing." It's "I want to explore a few different options for what to dive into, and then pick the tastiest option depending on what mood I'm in." (There is a shared assumption between these frames that there is too much information to consume it all. Do you actually just read every comment on a thread and find it valuable, or every comment in RecentDiscussion?)

The intent is not that you use single-line comments to do that. Rather, single-line comments are intended to make lower-karma comments fade into the background, so you can do a breadth first sample of discussions that are more likely to be interesting.

In the case of RecentDiscussion, one comment per post is expanded enough to get a sense of what's going on in the conversation, and you get to see if particular authors have commented that you tend to like reading. And if the most recent posts are all things you're not interested in,

Restaurant metaphor

To me, the default, fully expanded version of RecentDiscussion, or posts with big comment sections like this one, feels like going to a restaurant where there is no menu. The way I find out what's available to eat is to walk to a table that has the first item on the menu, decide whether it's good enough to stop and start eating. And if not, walk to the next table and do it again. And repeat for 100 tables.

Whereas a good menu a) lets you peruse a high level of what sort of items are available, b) usually shows you a picture of the items that are particular specialties of the restaurant in question. And then if none of those appeal to you you can read through the rest of the list.

One thing that might not be obvious is that, on posts with 50+ comments, comments that are unread-since-your-last-visit are fully expanded by default. The assumption is that you're either arriving at a giant comment thread where it probably makes sense to first glance at the 4 most upvoted conversations (which might be deep in an otherwise mediocre comment thread), or you're catching up on existing discussion (whereupon getting to the latest comments probably makes most sense)

(by contrast, collapsing things as I go doesn't help with any of that)

Seeing half a line of a comment is usually not enough information to decide whether reading the whole thing is worth while

I want to argue that this is a huge problem with the way people write here. If I have to read the whole comment to find out what the whole comment is about, that really limits the speed at which I can search the corpus. Sometimes, not only do you have to read the entire comment, carefully, you then have to think about it for a minute to decode the information. Sometimes it turns out to just a phrasing of something you already knew, in a not-interestingly different form.

If you don't make a body of writing easy to navigate with indexes and summaries, people who value their time just wont engage with it. They wont complain to you, they'll just fade away. They might even blame themselves. "Why can't I process this information quicker", they will ask. "I feel so lost and tired when I read this stuff. Overall I don't feel I've had a productive time."