For LINDEGR: It's been my experience that the FC and B/E cabins are always full (except on a few touristy routes), and hordes of people board when medallions are called without even trying - presumably LTUs - so the chance of an LTU making it is very, very small. Yet hope springs eternal, I suppose. Strict logic would indeed dictate, as you say, that LTU fares should remain non-upgradeable, but let's keep it simple.