At 18, these kids can do whatever they want. If they choose to participate in NCAA athletics, they have to comply with the rules in place at that time. If they don't want to comply with those rules, don't participate in NCAA athletics. It isn't compulsory.
I'm not suggesting that the rules couldn't be changed to offer the athletes some compensation - just that the athletes who voluntarily entered into an agreement knowing they couldn't receive compensation shouldn't expect to receive any.
This is true. If college athletes want to get paid and are able to do so, they can always choose to go directly overseas (or stay domestic, depending on sport) and work their way into a league. And if the NCAA wants to have college athletes not get paid, and "only" earn their scholarships in return for the entertainment they provide, they can refuse to pay them. But at some point, there are laws about whether an organization is required to share profits with the people "doing the work". And there are laws about whether an organization can impose whatever rules it wants on its members. Antitrust laws may also apply in the case of the NCAA, if there is no real, reasonable alternative to the service/product they offer.
But the real bottom line is that keeping the current arrangement viable -- where not-quite-ready-for-professional-career athletes have a place to get coached, enjoy competition, continue to hone their skills and bodies, and get an education, and where schools and the NCAA earn money, provide entertainment to their student bodies and the public, and generally build their brands -- is likely the best outcome for both groups. When one group feels treated unfairly, the arrangement changes to accommodate, or it breaks. As of now, it looks like it's changing.