This isn't really right. At all. I'll stick to basketball, but I'm sure that you could make the case in other NCAA sports, too:
High school-aged Americans don't play in Europe because they aren't good enough, they aren't mature enough, and because don't have passports. The latter is especially important because a lot of domestic leagues have rules limiting international players on rosters. Plus, people who make the kind of claim that you're making take the level of competition for granted. Brandon Jennings and Jeremy Tyler weren't remotely good enough for even mid-level European basketball. Most of the American high school washouts that go places with decent basketball leagues find that they're simply not good enough to compete. I'm not sure that playing in Qatar or Panama, among other places that host shitty basketball leagues, really complicates this point. Similarly, the "paying nice salaries" argument is crap. Some players make good money, but a lot of these teams pay shit wages and are really inconsistent in terms of actually paying their players. Regardless, nobody pays as well or as reliably as the NBA pays its athletes.
The main reason why, at least in my opinion, that there is not a viable alternative to the NCAA is that the NCAA has a monopoly on exposure for college-aged basketball players in the United States and the D-League cannot compete in terms of player development or basic human comfort (the salaries are pitiful). If professional sports teams in the United States took the European "developmental" model seriously (or the academy model, more generally), then I think you would see very different trends. It wouldn't eradicate the NCAA, but that's not what deacphan is talking about at all...
When you let the free market work itself out (pre-age limit NBA), then you'll find that the best players simply do not need the NCAA. Though, as is pretty clear by the NCAA and its school's reliance on stars to build brands and generate revenue, the NCAA and its member institutions absolutely need the best "amateur" players. That alone, IMO, is the case for paying players; with very few exceptions, they generate all of the value.