I don't quite understand what you mean by Gmail having no way to locally archive email. I use POP to locally cache all of my email on all my computers.
But the little bit of extra work one has to do in Gmail to get local access to your mail is simply one trade-off. The real point is that using Gmail involves a variety of trade-offs, which many email users may not be able to or may not want to make. So "just use Gmail" is not a sufficient answer to the problem of spam.