that's the whole issue. the NCAA has created a thriving black market in athlete services. black markets only exist when the official system contains significant market failures, in this case, because the compensation officially offered is far less than the market value of the services, at least for some athletes. people love to argue about how athletes are getting such generous packages, and that is not wrong, but if the official package were equal to the market value the black market simply would not exist. therefore the talking point that "athletes are already getting plenty" is demonstrably false, at least in an economic sense.
in my view the Title IX and other issues inherent in schools paying athletes directly may be too hard to overcome (these are distortions in the market artificially imposed by society, often for very good reasons). however I can't conceive of any rational argument against allowing athletes to hire whatever business advisors they choose to help look out for their best interests, including agents. To me that is a basic free association right and I don't see how the NCAA and the public school members, at least, get away with restricting their students' rights in such a way.
likewise I don't see a rational argument against allowing athletes to receive benefits from outside the school based on their image, or from a part time job, or whatever. If Don Flow wants to give our whole football team phantom jobs in his customer service department, fine. He has to explain that to his board and he has to pay payroll taxes and social security on the money, and the players have to withhold taxes, so if everyone involved agrees to the deal I don't see the justification for the NCAA to say it can't happen. The only argument I hear on that boils down to a fear that the big money schools with the big money boosters will pay athletes and get a huge competitive advantage against smaller schools (i.e., WFU) who don't have as many big money boosters. If that's your argument, I got news for you - WFU is already not remotely competitive with the Alabamas and tOSUs and so forth under the current system, and never will be. We all know those boosters are already paying players under the table. Instead of allowing a black market with the associated harms to exist, just bring it into the open and allow athletes to get paid by whoever is willing to throw money at them.
People used the exact same arguments about the amateurism requirement in the Olympics. They were bullshit in the Olympic context and they are bullshit here, too.