How could it be disallowed? I haven't seen any NIL legislation with caps.
There are two potential factors that could provide an avenue for limiting that.
One, the fact that the Supreme Court teed off on the NCAA does not blow up the NCAA’s ability to establish rules entirely. They can still make some rules provided that they are carefully crafted to be reasonable, very limited in scope, etc.
As an example, an NIL policy that permits athletes to enter into any deal they wish, as long as that deal serves a legitimate commercial purpose may not be an issue if that’s as far as it goes. It’s not necessarily off the table for them to state that recruiting inducements disguised as legitimate endorsements are prohibited and that schools would be subject to penalties if a booster were caught.
The second factor is that people who are exceedingly wealthy also tend to be pretty risk averse. And unless and until the Supreme Court grants relief to Jim Gatto, et al, then there remains established precedent for the FBI to pursue criminal charges against those who try to skirt NCAA rules prohibiting recruiting inducements.
If the NCAA decides to include some language that speaks to inducements being prohibited, then any booster who tries to buy a player via an endorsement faces the possibility of prison time if caught. The majority of wealthy people don’t like to take those kinds of risks.
And for the small percentage who are comfortable with that risk, there’s a decent chance they might already be cheating today anyway.