That wouldn't protect you in the scenario where someone had limited physical access to your machine.
Edit per the response:
I don't think this is a threat model that you would care about in theory, but it seems like a scenario that can come up fairly often in the real world. Especially in family or school settings, someone else will often have short durations of access to your machine. These people aren't often going to be technically literate enough to install key loggers or really mess up your machine, but they may well be able to quickly set up forwarding of your email to their account.
True. Alternatively, given they know how often a person checks their email, they could show the warning for some multiple of that time (with a lower limit e.g. show for at least 1 week).
Edit per the response: I don't think this is a threat model that you would care about in theory, but it seems like a scenario that can come up fairly often in the real world. Especially in family or school settings, someone else will often have short durations of access to your machine. These people aren't often going to be technically literate enough to install key loggers or really mess up your machine, but they may well be able to quickly set up forwarding of your email to their account.