The problem for the Colts has always been that no-one who's any good wanted to be the backup to Manning, because they'd never get a game. If they do draft Luck then you might not see Luck play for years. My guess would be they'd keep him on the bench for at least 2 years and then try to trade Manning. The Colts might be better off waiting 2 years for the next QB and using the time to address, well, everything else in their team; the drop-off in performance without Manning has shown just how good a QB he's been - makes you wonder what Manning's stats would be if he'd ended up with the Patriots for example.
Rather than focusing on the one player for a future QB I think we just need to focus on getting the best QB we can without sacraficing too much - maybe the top two picks in 2012, plus some pick from the following year. A top 5 QB surrounded by a good team will do a lot better than the number 1 QB forced to carry his squad.