I keep saying this, but it's like no one is paying attention: good luck enforcing that in a court of law. It is *extraordinarily hard* in this country to go into a court of law and *prove* that a transaction with a willing buyer and willing seller did not occur at "market value." It becomes even more difficult when there were multiple bidders for that same "service."
Basically, if a random linebacker gets an offer of $500,000 for signing 10 footballs from a money U donor, and he also has an offer for $450,000 for signing 10 footballs from an oil baron U donor, good ****ing luck convincing any court in the US (any *real court*, not the kangaroo ncaa committee "court" they're setting up) that $500,000 for an 18 year old no one has ever heard of before signing 10 footballs is not "market value." Multiple willing buyers, and a willing seller with a bidding process (open or closed bidding process doesn't actually matter on this) setting the price.
If you haven't noticed, the NCAA can't get anywhere near a courtroom without getting beaten like a rented mule for going on about 10 years now, and this is an issue on which they'll lose so, so badly competent lawyers would be embarrassed to even represent them (way too much case law on market value transactions in this country - we are a capitalism above all else society with the legal system necessary to support that, and this is a core principle).
There are certain instances where we can try to place a fair market value on certain things. Businesses - we can look at the underlying assets, profit margins, and revenue. Real estate - we can look at comparable sales. Some services - we can look at the compensation of other individuals providing similar services. Happens all the time in tax cases, divorce, etc.
But here - it seems impossible. If other parties are willing to pay the same compensation, as you noted, it's hard to argue that that a college football player is being paid in excess of the fair market value. And you have instances where professional athletes make millions and millions in endorsements. "I thought player X was going to be the next Micah Parsons, or Patrick Mahomes, or LeBron James." Seems totally unworkable to me.
And maybe that's why, in the articles I have read, athletic directors and people close to the industry speak as if there is no cap at all.