I've been watching soccer intensively since the end of the 90's and I'll say that the US quality of players has increased. Obviously as with any sports, results will be volatile because the quality of the opponents is always getting stronger as well but with the increase in popularity in the MLS and the increase in interest by fans, the sport should grow over the long term.
Maybe I'm a bit biased because Bradley plays in Roma and he has obviously had the luxury of learning from one of the best in the game in De Rossi but I really do think the Americans will get there. That being said, it is a difficult tournament and it was only 10 years ago that Spain was irrelevant. Things can very easily change.