This seems like the sort of thing that should be in the spec for a feed. RSS is basically a textual representation of a site's available articles; you have to explicitly GET the feed to see any new posts (at least without PubSub). If you're going to provide a static xml representation of your site's content, there's no obvious reason why you wouldn't want that to extend back indefinitely.
By looking into Podlove[1] I stumbled onto a specification[2] by the IETF for just that. It doesn't seem to have much support though, neither on the feed creator, nor the client side.
Something like feed.xml?page=2 seems fairly sane.