I'm fine with the way it is now. I do sometimes go back and edit comments, but this is almost always because I messed up something I didn't notice until the final rendering. Comments don't have previews and the little edit window is smaller than a max size comment, so it's easy to miss something. 5 minutes is plenty of time to give it a read-over and fix typos. After that it should be frozen.
You should be willing to stand behind what you say and have a record of it be kept. If you don't like that, don't write comments. If you change your mind, write a new comment. It can be useful for others to see the evolution of your thought process as it changes. I find this less onerous than the record left with a strange comment that makes no sense and possibly looks stupid because its context was deleted later.
As for freezing a comment when someone replies, how does the system know until someone submits the comment? What happens when the two are edited simultaneously? The system can't always know when a specific comment is being replied to.
There isn't a problem here. Let's not fix something that isn't broken.