PHPBB's paging system was trash, Discourse's "infinite scroll" system is only marginally better... from a visual candy point of view; for discoverability, it's still trash.
Nested threads are a thing since email, and there's a reason for that. Even Mastodon, a "Twitter-like", has them. Heck, even Facebook has them (to a degree). The concept could still be improved upon, but going to a non-nested solution would be a serious step back.