Actually, I expect there to be a dramatic increase in American football talent over the next generation for a variety of reasons:
Youth systems are becoming much more sophisticated and better funded (with many now establishing affiliations with European clubs).
The Central and South American immigrant populations are booming and increasing the profitability/competitiveness of the sport (in fact, the MLS just recently signed a major television contract with one of the major American broadcast networks).
The MLS, while still relatively terrible, is improving and the best young American players are learning to play alongside the likes of Beckham and Thierry Henry.
Most importantly (IMO), with the arrival of digital cable and the ready availability of dedicated football channels, middle and working class American kids with an interest in the sport are watching and learning from European leagues every week, whereas just six years ago, the World Cup was the only thing that all but the wealthiest Americans really knew about the game. Now, young American football fans are growing up with players like Messi as their role models and understand that "soccer" is more than just the "kick and run" game you play before you're old enough for the baseball team.
I'm not saying there's going to be an American Messi anytime soon, but the football subculture in the US is definitely on the rise. It'll likely never eclipse gridiron or basketball, but in a country of 300,000,000, a sizable and growing subculture is going to be producing a lot more top flight "CA 155" players in the mold of Clint Dempsey and Tim Howard.