I'm legit surprised that that isn't Teach's way of thinking about this...
I can only assume it's because he doesn't have any faith or motivation in himself to believe it's possible.
After all, he still doesn't know just how or why he's important. And that's pretty important information if you're going to tell the entire town "I can fix him". Especially when subconsciously, the guy still wants to kill him.

Convincing Guwon through the teach's help to change the rules is the only way to live on bcoz if they were to kill Guwon, not only will they not receive protection but also it will eventually kill all of them due to limited resources.So it's like leaving one hell js to end up in another hell.