That’s the main goal of the Summarize button – to take a very long discussion and distill it down to the 10 percent best and most relevant posts (as measured by likes, replies, quotes, read time, etc).
Now this button won’t appear until the topic has > 50 replies, and it is a “best 10 percent” so you are still looking at minimum 5 posts in a 50 reply topic, 10 posts in a 100 reply topic, 100 posts in a 1000 reply topic, and so on. But they are objectively the best posts.
And you can dynamically expand and contract the replies to those best posts like so:
This is how Discourse is a hybrid lightly threaded model.
Always a good time to remind people they can click or tap the reply icon in the editor to convert to a new linked topic:
It’s sort of the same question we’d ask Google – how many people really go to page 2, 3, 4, 5, 6, 7, 8, 9, 10+ of the Google search results when they are searching? I know for some more difficult stuff I am looking for I will definitely go to about page 4 before reformulating my query, but I doubt I’ve ever been beyond page 10 of the Google search results in my entire life.
It’s fine to have a few larger topics, but the most important thing is to encourage creating new topics rather than piling on a small number of infinitely large topics. That’s basically .. chat.
In my experience threading doesn’t solve anything in particular – it creates a million-headed hydra of infinite stuff to read. The problem isn’t so much “this would be comprehensible if threaded” but “ain’t nobody gonna read that many words”. 
A couple options beyond summarize come to mind. First of those is searching within the topic like so. If it’s a long topic and you only care about the word “hello”, then search for all the posts that contain the word “hello” in that topic.
Second is reading the last 100 posts, easiest to do by entering the topic at the bottom, by clicking or tapping the post count, or last reply date marker, like so..
.. and scrolling upward from the bottom. You will possibly miss some earlier activity, but the idea is that the last and most recent activity is likely the most useful and relevant.