There is no reason pay has to be flat with a union. The highest paid members of MLBPA or SAG-AFTRA are paid 1,000x what the lowest paid members are.
Interesting point. I think there might be something special about sag given the way Hollywood does business. Everyone switches employers constantly as projects spin up and wind down. Having a mostly permanent role in an organization is super unusual - and I wonder if these roles (like a 20 year soaps gig or the simpson’s) have different stats on working on/off the card.