It wasn't GET mutation... it was POSTs with multi-page forms that was the problem. It was such a pain to subdivide a form and create server and session state and intuit the return state. And what happens if you needed a modal with dynamic data? Did you pop open a new window and create a javascript call for the result? There was no great progressive answer to them.

Oh, and then request scope wasn't good enough because you needed to do a post-redirect-get? I will say that I do not think MPAs for web applications were the good old days.