You can change if you're replying to root or to the post specified inside the editor.
Is that not discoverable enough?
Apparently not, because the mistake gets made often, along with the lamentation that "damn it, why can't I edit that?"
The problem with defaulting it to the root all the time is that then there will almost never be anyone that replies to a person specifically. I would challenge your assumption of "Most ppl are generally contributing to the conversation, to to one direction person". Can you prove that, because I think the opposite is true (hence our current design).
I'm not asserting that at all. I'm asserting that most people expect a 'reply' button to be a 'general' reply. The quote button, for instance, was a much more explicit way of saying "I'm replying to this specific guy that I'm quoting."
Take this thread as an example. Sampy is the only one that replied to the "root". Everyone else replied to a person in the thread (and I believe that was their intentions).
It's a tough one to prove either way. You can't just go through the data, you have to read through each comment and verify what they really meant.
Again, I'm not saying that most people want to reply to the thread. I'm saying that there's confusion. And I feel I can assert that because I'm seeing "Damn, this should have been a reply to the thread, not to randomguy" posts quite often. The reason for this confusion, I think, is that "Reply" is too vague. It's open to interpretation. And since "reply" in many forums means "reply to the thread", I'm guessing people expect that to happen here too. If it's paired with a "quote" or "Reply to guy" button, it's intention is much clearer.