Of course, you also have to take into account the fact that more money is poured into the European leagues now (and even China is getting into the action). This isn't to say that potential NBA superstars are staying in Europe rather than playing in the NBA, but it's definitely not a clear-cut decision for players to leave Europe for the NBA. Rubio stayed in Europe for a few extra years even though he was a high draft pick. Tiago Splitter was originally going to declare for the 2006 draft, but NBA teams balked at paying his buyout. Then, instead of coming over from Europe right away in 2007, he stayed in the Spanish League, making eight times what he would make as an NBA rookie (San Antonio, always masterful at managing cap space, figured this was a possibility). (All this info is from
his wikipedia page) Andrei Kirilenko even left the NBA for a while during the strike-shortened season because he could make good money in Europe.
This isn't to say that surefire stars are staying in Europe instead of coming to the United States, but it does seem that it's getting harder to get prospects and established players who are unlikely to be stars in the NBA to take less money to play in the NBA when they can make more in Europe. While the NBA might be lacking a little bit in fresh international star power at the moment, I think that's more of a fluke than anything else. Of those guys in the first list that you made from the 2000's, only Dirk is a surefire Hall-of-Famer (Parker and Ginobli will likely be knocking on the door and Yao probably would have been if it weren't for his unfortunate series of injuries). That no one else has had that type of career doesn't necessarily speak to a lack in the over all talent level of international players (Horford may not be Dirk, but when he's been healthy he has been outstanding). I would be willing to bet that European teams, as a whole, are generally deeper and more talented than they were 20 or even 10 years ago, it's just that a lot of the guys who would be rotation players/end of bench guys in the NBA are staying in Europe. Remember, the Greek team that managed to hand Coach K his only loss as the head coach of the national team didn't feature an NBA star. I would argue that the Spanish teams that the USA has defeated in the gold medal games of the past two Olympics have actually been the best international teams the USA has ever had to face, including Argentina from the 2004 Olympics. Still, the USA has emerged victorious.
I think the USA's winning streak has more to do with the fact that the USA still has, by far, the best talent pool. The losses in the early 2000's and even 2006 can easily be explained by the rest of the world catching up a bit in talent level and the fact that the USA's basketball program's philosophy was basically roll the ball out there and see what happens ever few years. Even with the best players, other countries feature enough depth of talent to beat the USA if the USA doesn't prepare properly. As we've talkied about before, Coach K's and Colangelo's greatest legacy will be the establishment of a program designed to develop the national team beginning at the youth ranks. As long as that commitment continues, I don't see the USA losing its place as the team to beat anytime soon, even if we do lose a game here or there.