I think we’re just teasing at the notion that “permanent failure” isn’t a hard and fast distinction. I think some polite retry policy is not unreasonable even for the most explicit “permanent failure” response from a remote server. Imagine the most extreme example: hackers take over the remote server and make it respond with “permanent failure.” After a day, the legit owners regain control of the system. You can’t really argue that “the remote server never should have delivered that response unless the failure truly was permanent,” because clearly there was a mismatch between the apparent intent behind the response and the actual intent.