At this point there's a decent installed base of automated bill-reading machinery, though. Not that these ought to be sacrosanct or anything, but the cost and inconveniences associated with a rollout of new bills is substantial.
People will complain about anything. Americans have very odd sentimentality towards their notes and coins, as if history itself will be rewritten when a change is made in the present.
The classic example is the penny. Get rid of it! You can still collect them and read about them on wikipedia, you just won't have to deal with them while buying your double cheeseburgers any more.
Plus, people will complain about the new ones. :)