They don't have to join the union, but the union agreement would set parameters for length of eligibility, amounts allowed to be paid by the school or their affiliates, etc.
A player cannot play forever just because they are not in the union.
There are 2 kinds of NIL:
The endorsement/personal appearance revenue - what NIL was supposed to be
The pay for play revenue - what it has become in the majority.
The union bargaining would only cover the second (I would hope). Why would someone who hustles and gets a commentary gig during the...