Why did they retire the Concorde???
Simple, ECONOMICS
The Concorde was always more of a prestige thing for BA and AF then a money-maker. Remember the Concorde was ALL first class, and with tickets costing much more then regular first class tickets, they didn't sell a lot. (granted with the expensive tickets, if they had sold out every flight it would have made a lot of money... but they didn't) Next factor was the ever increasing cost of maintaining the aging fleet, also they had much higher maintenance costs to begin with.
This talk of it being more efficient is total malarkey too, the Concorde burned more fuel crossing the Atlantic then a 747, while only carrying 1/4 the passengers (hence the super high ticket prices) ... the old saying is true "Speed costs money, how fast do you want to go?" As a general rule of thumb, you'll always burn more fuel to go faster.
Bottom line is $$$$$$$... the Concorde wasn't making it.
EDIT: I should add that I would LOVE to see a modern supersonic airliner, the technology is there, and they can now muffle to a degree the sonic booms. Making it possible to use it trans-continent and not just trans-oceanic. I'm not holding my breath though, most airlines are still struggling just to stay in the black, I can't see any of them taking a risk on something like that. Not to mention it would take a decade to get such a plane through FAA and EASA certification even after it was built.