unless someone explicitly asks for their account to be closed / removed / the forums completely go offline, all accounts which are still active will remain active to my knowledge (especially when no one has the time to prune old accounts anyway

).
though i'll note that if someone did request an account removal and has a large number of posts, removing an account then becomes a pita as it causes no end of trouble to keep thread flows correct, etc - as i'm finding at the moment from a request i'm processing.
-daz