If you want it to be the only copy and not sync with anything

POP3 is line–based too, anyway. Maybe you can rsync your maildir?

Isn’t the only difference between pop and imap that pop removes the mail from the server? I only use imap, and all my email is available offline.

But it's more akin to consuming a message queue. You have fetched it, it's gone.

Nothing stops you from locally archiving your email with IMAP.