Define pro? They're getting paid now.
That's why I said former, and it is a relevant delineation.
Logically, no one wants former NFL, MLB, or NBA players playing in NCAA sports. That makes no sense. But the problem is it also makes no sense to limit it to a few select pro organizations. It has to be all of them. That includes minor leagues, G-League, European leagues, etc...
Specifically if you got a paycheck to show up for work in another organization, you're a pro athlete (same distinction still exists in golf for example, there are distinct amateur and pro classifications). Now, I was anti-NIL, adamantly, I said this all would happen, I said players would sue for eligibility, we're here now, it is what it is.
But, if you don't hold the line there, then you really do just let the NCAA become another minor-league. It has to be two distinct eco-systems. The pro sports and the well you can call them former, amateur sports. Otherwise, if a guy gets cut from an NFL team he can just go back to college. But the same would apply for the CFL, the IFL, etc... and then you have utter chaos.
The age limit is one way to handle it, but it seems almost as messy as what we currently have. There just needs to be clear, unavoidable limitations. 5 to play 5, can't have been paid outside of the traditionally amateur classification to play the sport. To further that distinction you'd probably have to specify which is which. For instance, college and high school, former amateur model, IFL, CFL, ELF, NFL, UFL, professional model.
I don't think people really want Chad Kelly to leave the Toronto Argonauts and play for Ole Miss again. The age limit is a way to prevent that, but it doesn't prevent a younger player from doing something similar.