Sorry 1miau, I agree with @hosseinimr93 here: if one posts into a self-mod topic he knows that his post may get removed. This doesn't happen with a "free" topic. Changing this "during the game" is unfair advantage for topic owner.

Just imagine a shady business getting a couple of bad reviews: in your proposal it will be allowed to change to self-mod and remove those. Those users, if they would have known that there's a chance their posts get removed may have been made the complaints in completely different topics.

Your use case takes into account only your situation when the topic starter is honest and the posts he wants to remove are probably spam (which can be reported) or bad intention garbage (which, at an extent can be ignored, and if a threshold is passed the topic ends with a link to the new moderated topic). We are old enough around here and can find our ways. Let's not give more tools to the scammers only because we want some things be easier for us.