My mail stays on my computer, backed up locally and encrypted to a remote location.
I lose the ability to access old messages from remote but it never was a problem in almost 17 years of self employment. Before that, I already had about 13 years of downloaded emails, from the times when web mail was not a thing. No need to upload my old messages to Gmail, if this is even possible. I just kept using a local mail client. For my personal mail I remember some emacs clients, Netscape Mail (bundled with the browser,) Outlook Explorer then Thunderbird. I might forget something.
Yes, before I ran my own mailserver, I strongly preferred POP3 for this reason -- it let me get my email off of a server that I had no control over and onto a machine that I do have control over.
But after I started running my own server, I prefer IMAP so that all of my email is in one place regardless of what machine I'm using. And it still lives on a machine that I have control over.
I'm curious as to why, if you don't mind! Are you specifically looking for "download only once then get rid of it"?