Good points being made. I think that crux of this suggestion is, if I had to give it a name, Respect The Scope. This is to say that when somebody puts effort into creating a new thread with topic they want to discuss, all posters should respect and consider how their reply fits into the scope of the discussion.
Maybe the way to implement this not to explicitly disallow any type of post, such as refuting the central premise. Instead it could be recommended that any reply to a thread that might goes outside the scope be prefaced with something along the lines of "not trying to go off the rails here" or "sorry break up the discussion." And if this scope stretching post isn't resolved fully in say 2 or 3 replie, it would be time split the discussion. Preferably one of the replies would recognize that the side conversation is overshadowing the main scope and invite any follow up to be in a separate thread.
Under this rule my drafting example might look like this:
This drifts a bit from the initial proposal as isn't as hard and fast of a rule. But it does avoid invalidating criticism on grounds that the criticism wasn't allowed to be made.
I might just be saying some stuff that seems like a no brainer, but if we're not doing it too often right now, it would be worthwhile to add it to the Guidelines.