I would agree *if* that was the only axis that the two tools differed on. (Un)fortunately there is a laundry list of improvements over the traditional mailing list this is available in modern mechanisms for facilitating discussion that mailing lists lack. Even something as simple as being able to decide on a topic by topic basis whether you want to receive emails on that topic. Unfortunately it’s hard to impossible to retrofit these items onto email, because email has a lowest common denominator problem where you can’t meaningfully improve it, because you can’t break support with the huge deployment base of every mail client ever.

If there were a system that offered even most of the benefits, but allowed someone switching between tree view and flat view, I’d be all for it. However all of the modern systems I’m aware of only allow one or the other.

To be honest, I’m not even sure how you’d represent some of these things in a threaded view. For instance within discourse I can multi quote different posts to tie multiple lines of discussion together. How would you present that in a threaded view? A merge? I’m not aware of any threaded system that actually allows it.