Responding to some comments from @codinghorror over in the originating thread: How difficult would it be to make the Discourse UI more like Flarum?
The “text input box” (not really a text input box) is only at the bottom of the topic. It does not replace all reply buttons, only the last one that in Discourse is highlighted in blue. I think what it does is encourage response, but not premature response.
As a “very online guy” myself, I seldom have trouble feeling confident to respond, but surely you know that many people do. Initial participation in forums can have a high barrier to entry, and this is one of the things discussed (and not yet resolved IMO) in threads like this one: Circle.so vs Discourse
Obviously you want to avoid the problem you raised: people responding too quickly, knee-jerk, etc. But hopefully we can trust that by the time they reach the bottom of the topic, they’re ready to respond. Or at least, Discourse already seems to trust that, so… 
Depending on how this will work, the text box at the bottom of the topic might make even more sense then…