Sure, but the friction remains the same under this idea until you reach the end of the discussion. Yes, it modestly decreases friction for a reply at that point, but not in a functional how-many-actual-clicks way, only perceptually.
More importantly I don’t think a “Someone is wrong on the Internet!” rage-poster is going to be much affected by the difference between these two approaches, whereas someone who is anxious about participating might well have a more measurably different response to the two. I don’t have studies to back this up (if you need some, maybe I can find some though 
), but it at least makes intuitive sense to me. And ultimately, IMO, is worth trying because I think you’d quickly see what the results would be in any decently active forum.
I’m still confused. The “reply text box” feature requires you read all of the topic before you see it.
That said, I get it, this is something to try to do in a theme component. So, any takers? One of these days I’ll just have to bite the bullet and learn how to program for god’s sake. But right now a bit too much on my plate unfortunately. 
Also, you and I have potentially different views of how chat should be handled. If there is a way to preserve valuable content from chat, then by all means it can be ephemeral. But I’d want at least a week, if not longer, to determine that (i.e. chat retained for that long). Ideally that will be admin-controllable. But I digress from the topic of this, er, topic. 