[mindslight]

You could write the exact same comment about USG, and yet here we are...

"The Market" (or however you want to think about it) is not perfectly efficient - if it were, there would never have been any motivation to form unions because every employee's requirements would already be perfectly reflected in the companies they chose to work for. Rather, sluggish network effects and other non-invisible-hand structures dominate immediate results.

All power tends to concentrate. When it gets large enough, it attracts those who seek and hold power for their own ends rather than any benevolently stated goals. Your scenarios are straightforwardly mitigated by union leadership directly appeasing the leaders of such movements, the same way monopolist companies buy out possible competitors to keep a stranglehold - the creation rate of competition is itself limited.

I don't know too many specifics about SAG-AFTRA itself, but its hold over the entire industry is a good indicator that it is indeed too large. Its size implies politicking on the order of at least a city government. Conversely, that size may also be the minimum necessary to engage in collective bargaining across different production companies, and so any general top-down reform is a bit hazy. My only direct experience with this is seeing casual SAG actors having to do a bunch of extra paperwork to participate in a strictly non-profit hobby short film production. It seemed quite ridiculous and onerous, until I thought about the neighboring gradient of low-budget for-profit film production.