It's not a zero-sum game. A company with happier, more productive employees under a union may have higher costs, but their odds of putting out a more-successful product are also higher than a company that burns out and/or screws over its devs.
Union workers, including professionals, across the U.S. report considerably lower trust and comfort in the workplace, and feel more distant/guarded from their bosses than non-union workers in every sector [0]; and I'm not exactly sure how you measure productivity, but I can't exactly assume that that's higher in a union shop either (anecdotally, it is the exact opposite of the case).
Now, some of this might just be a matter of unions being more common in crappier industries, which is plausible, but it certainly doesn't indicate a positive impact of union membership.
[0]: https://news.gallup.com/poll/146687/Union-Workers-Score-Lowe...
I don't see what the union would accomplish aside from employing union leaders and sinking companies which allow it to form. The market is competitive enough that if a major studio has any considerable union power, the firm will most likely just lose to more efficient companies, due to cost.
I was fired once (according to people who heard from him about it, so grain of salt I guess) because the CEO thought I was attempting to organize the workforce at his (horribly dysfunctional) company, but I can't really see how organizing those people under that particular company would have done any good.