I think warning emails would quickly become overwhelming, both for the server and the users. Another feature which I requested in the past is to sort feeds by “oldest unread”: Sort feeds by most-unread and oldest-unread.
+1 for increasing the expiry time, but my experience has been that many stories are expiring far earlier than 30 days. See my post here: The Dreaded Ten (ie, aggressively deleting feed item)