The flat sequential thread was a reaction to threaded messages as seen on Usenet and mailing lists. It works reasonably well if users are careful to quote the messages they're responding to.
https://en.wikipedia.org/wiki/Conversation_threading#/media/...
VS
https://preview.redd.it/r7k2gf1kav6z.png?auto=webp&s=9fd7a0b...
I think the flat sequential thread works fine with a limited number of messages. Eventually with enough replies it degrades to being very unwieldy like traditional forums.
In fact here's a screenshot from 2005
> It works reasonably well if users are careful to quote the messages they're responding to.
Unsurprisingly, they mostly weren't. Linear threads worked reasonably well when you were participating in or following a discussion as it unfolded; otherwise, they quickly became impenetrable.