That is an excellent point. However. It’s also true that we expect people to read conversations, and we expect positions to evolve in conversations through multiple responses. So expecting that someone has a single post in the topic with precisely and exactly their final understanding … is unrealistic.
I definitely hear you on the triggering XKCD 386 factor though. If the triggering bit can be edited out, then all subsequent replies will be better ones, and the whole topic improved … 
I want to reiterate that I am firmly a fan of “allow one edit”, especially in the case of “oh my, I’ve posted very dangerous misinformation”, but I feel the risk of this is being overstated by folks a bit, and they are understating the risk of people turning the topic into a live editing chat session… but allow one edit is a good middle ground here, too.