You’re right about the productivity. And many movie studios already own VFX studios. Netflix has Scanline, Disney has ILM, etc. But it is not super common.
I was more thinking about employment by the VFX studios themselves - i.e., if the article is right and VFX companies are already in a race to the bottom with respect to the movie studios (their customers), this race would likely swallow all productivity benefits that generative AI could provide.