Look at this conversation, it's old enough that it doesn't show in my feed anymore (sorted by top 6h), if I wasn't taking part in it no matter how many people replied to it I would never know it took place.

That's what I'm talking about, if sorting is up to the user then most people only see "fresh" content, not ongoing conversations that they might want to take part in if they realized they're happening. Same for the comments sections, threaded makes it harder to check what's new (have to go down each branch to get the context).