> Is there any reason why flat/tree view could not be toggleable

I actually implemented both flat & tree view in Talkyard (forum softw I'm developing).

Not toggleable, but side-by side, so you see the threaded discussion in the middle, and comments by time descending in a sidebar.

See: (incl demo video)

https://forum.talkyard.io/-32/how-hacker-news-can-be-improve...

(Old HN discussion: https://news.ycombinator.com/item?id=12663844, but blog link broken (seems I let the domain expire))