I'm opened to having my opinion changed, but I'm against NIL for competitive balance.
I'm all for the free market, but in a closed system/zero sum game (especial regarding sports as entertainment) you want competitive balance. NFL & NBA have salary caps so that small market teams (G5's & ~90% of P5's) can compete with the large market teams (~top 10% of P5's). This makes for a better product (NBA, NFL).
I know, I know, I could have added 20 caveats to the post above. But that's my concern in a nutshell. There's not much competitive balance as is, but now I think there will be even less.
P.s. As an aside, college athletes are supposed to be amateurs. They're already receiving a free education worth thousands upon thousands of $$$. Plus the athletes get a number of plush perks regular students don't.