The mockup is a good idea, but I think you could force people’s hand to click the arrow (via some complex, imaginary set of remote robotics), and it still wouldn’t change their behavior.
I’m … not optimistic … about the idea that you can get people to go out of their way to create new topics, even if they know exactly how to do it, when it’s so much easier to reply to the existing topic.
(I also think it’d be a mistake to make it very easy to spawn a bunch of new topics willy-nilly. That’s trading one set of problems for another.)
All you’re left with is arbitrarily making it harder for people to reply, to incur intentional friction on reply, which has its own set of pitfalls. I mean technically we want people to read the entire topic before replying, yes, so we could block replying until we know you’ve read “enough” of the topic? Or we could block replying (somehow…) until you’ve read “enough” of the source article that the discussion is about?
There is no clear win here. The A+ students who sit in the front row of class will create new topics and help split. Everyone else … kinda doesn’t care.