It's possible one could do that, though it would mean showing people pages after they have expired. However, there are other ways to get to an old page besides the back button, such as history -- and as I've pointed out, bookmarks. If there are going to be time when you want forms to be resubmitted with data, you want to know what the consequences of that are, and in particular if there are no consequences. One of the classes of side-effects I should add to the list is "login." If a form logs you in, that's a big side effect in some senses, but really a very minimal one in other senses.

Right now the distinction between POST and GET is arbitrary. Both are forms, they are largely isomorphic. POST just doesn't show what you submitted in the URL, and can be used for larger bits of data.