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So did vB 3 and Invision and MyBB back in the mists of time.

The problem with switching back and forth is that it fundamentally doesn't work, or at least only works if you're switching from nested to flat - the other direction doesn't.

The reason is that in a flat thread, the reply is always to the end of the thread, never to any of the leaf nodes, so switching from flat to nested means you inevitably have one big chain of reply-to-reply-to-reply that dwarfs everything else.

Or you do what Discourse originally tried to do, which was have its cake and eat it, and inevitably led to people thinking they were replying to one post but the software logged it as replying to someone else's. (It's been a while, maybe they fixed this since.)