I think the mis-misconnects (when the passengers actually DO make the flight) are the ONLY situations where it's reasonable to bump the Y upgraders back to coach. Presumably they only got the upgrades a few minutes earlier. Still, CO should do a better job of figuring out if a pax will make a connection, although I guess if it's an international-to-domestic one, there's no way of knowing how long it will take to clear customs.

But to think that a paid F passenger can just show up at a flight s/he is not confirmed on, and expect to bump even the cheapest of coach passengers who had received a confirmed upgrade, is just ludicrous.