I've got concerns about the NCAA trying to draw a line here too.
In terms of losing in court, it has all been questions of eligibility that are arbitrary or inconsistently applied to particular players. I don't think antitrust law stands for the proposition that LeBron James must be allowed to play in the NCAA after his career is over.
I think if they said something like, you have to be academically eligible, enrolled as a full time student, between the ages of 17 and 24, and you only get 5 seasons to play, I don't see why that would have any legal troubles, those being clear and bona fide qualifications that apply equally to everybody.
That's quite radically different than the current rules though, with American pros coming back being the most obvious new talent source. How THAT gets prevented if European pros are allowed in I don't know. Could major college basketball fold in under the NBAPA in a similar fashion to how minor leagues are now part of the MLB union?