I think it would be good to establish a more positive attitude towards spinning up new threads.
Afaik there's no strict real rule that says all discussion about a specific topic should stay in a single place, but it's quite common that people complain when a second thread is created.
Why "multithreading"?
Threads in Discourse are sequential lists, not trees — so discussions are often scattered and mixed with other (sub)topics. This is especially bad when people start loosing themselves in debates about details that might not even be relevant for the actual pitch. I think that's fine, but it would be nice if people created separate topics to discuss the importance of Newton for computer science, or how the world would look like if the IBM had chosen a Motorola CPU for its first PC.
Counter proposals
The Evolution forum could be a much nicer place if pitch threads would always focus on improving a proposal (without trying to wreck ideas). But when you bring together many people with conflicting goals and opinions, you end up with a "destructive" discussion easily.
Splitting could make the group of participants not only smaller, but also more homogenous, thus reducing friction.
Instead of pointing out why "the enemy proposal" is worse, the question would be "why is our idea better?" and ideally, we would end up with a compact list of advantages for each possibility, which then can be presented in the review thread.
Who owns a thread?
I don't think the author of a pitch should have special (technical) rights, but when a thread starter acknowledges a concern and solicits to move on, I'd consider to grant them the privilege of moderation. The goal should be to help, and it's up to the author to decide what is helpful.
Being supportive does not mean to stay silent when you spot a deficiency, though — I'd say the opposite is true:
You can only address shortcomings when you are aware of them.
Also, if I came to the conclusion that a pitched change could only work after someone solves the halting problem :-), mentioning that could save the author some work...
I'd be especially careful to have read the whole thread before expressing negative feedback, though.
But what if an idea is just bad?
Well, if a proposal is outright terrible, it will never pass review, won't it?