That's my understanding as well. A person doesn't have to join a union (but still get to be free-riders on whatever the union contract has in terms of benefits), but they are still governed by what the union has negotiated.
The union can’t negotiate the end of NIL though
The current court precedents would not uphold a union telling someone else they can’t accept a third party endorsement/compensation
You are correct that non joiners would be governed by whatever is negotiated and agreed upon. You are incorrect that the negotiations would be free to negotiate whatever they want and it would stand
If a players union negotiated with the ncaa to not allow 3rd party endorsements, this would be overturned almost immediately upon challenge.
I could be wrong in this. The Supreme Court could overturn all the precedents they have been setting the last couple of years and we could go back to 2019. I don’t think that’s likely though
They could negotiate some things. They could agree no NIL work during “work” hours
They could agree no NIL stuff in sanctioned gear
Etc
But I’m not sure why the players would negotiate that against themselves
If you want to legislate NIL you probably need congress to step in and mess with the tax structure to slow it down.
If endorsement deals for college athletes were deductible, the amount of money just slowed down
If they were subject to targeted taxes, that would slow it down.
Etc