Ah yes. Please tell me how real your totally made up assumptions are on how AI will develop.

if the AI companies get their hands on it.

AI already had their hands on the sum total of everything humans have created for the last 10,000 years and the best it can do is sometimes generate something mediocre 1/100 times. This mediocre result can't be modified, or improved in anyway. And you want me to believe adding a single other library of 300k films will be enough? That's a bucket of water in an ocean of information it already has access to and uses poorly. It will not make AI magnitudes better than it is now, the technology itself has plateaued due to its own architecture. AI Models can get better, yet results still cannot be modified, duplicated, or improved.

Which in an industry where tiny creative iterative improvements is literally the key to making something great, you want me to believe a machine that is incapable of that can somehow reach the same quality result?

You are dillusional. This tech is a dead-end creatively and it's an embarrassment that you think it can grow by impossible leaps and bounds that exist purely in your imagination.

the outcome will likely be good enough for the average Marvel movie.

Dude. Marvel isn't even good enough for the typical marvel movie these days. You think the guys who made 26 blockbuster hits can be replaced by AI when they can't even successfully replace themselves with other humans? Marvel can't even make a movie to save their own ass these days, I doubt AI has a shot.

1 scene at a time means AI has results with inconsistent locations that change as they leave the frame, inconsistent costumes on actors, inconsistent background SFX, inconsistent lighting, inconsistent cinematography. Etc etc.

AI doesn't know how to keep the color temperature between indoor lighting and outdoor lighting consistent. So it has no control over style, rather just a single scene's lighting. So it can't be a cinematographer. It can just guess what each scene looks like and you better hope shot 5 is the same 5400 degrees Kelvin white balance as shot 3 or that scene is going to look like a fucking amateur shot it once edited together.

AI can't emote. Period. In any way that makes a human interested instead of angry. Acting is a range of emotions, displayed in ways uniquely and handled by each actor. An AI can give you 100 emojis options to have appear on a face in a scene, and none of them consider the emotional arc and narrative of the film, because this is one scene being generated out of 73. So AI can look and sound like Natalie Portman, but be limited to acting like her in Star Wars episode 2: Attack of the Clones. Just constantly one emotion full throttle or another to the point it's hilarious an Oscar winning actress gets completely forgotten about in her Star Wars appearances.

AI can't be consistant. Period. So sets will change. Props will change. And "plot holes" or "set mistakes" will become common as AI can't remember what something looks like when it leaves the screen. It'll be like accidentally having a Starbucks cup in that one episode of game of thrones, but for every episode of ever, instead of just the one in season 8. Imaging that seasons mistakes but done in every shot and every scene because the robot dreaming up the production cares even less than the directors did making that season.

People like you think AI can make a movie because you've never made anything creative yourself to sell to the public. It's not easy. And the public doesn't just lap up bullshit. There's trends and tastes and opinions that every creator whose ever made something has had to navigate to become successful. Just because an AI can get criticized instead of a human doesn't mean that AI can get better as a creative generator. Only humans can do that through constant iteration. AI, even at its best, still needs 100 tries to get something alright. An expert filmmaker needs 1. Because they've made thousands of mistakes before, and learned from each. Something AI literally is incapable of and will never be able to replace. So every creative human work that is built atop the journey of failure and self improvement can never be emulated by a machine that fully lacks that mechanism. At best, it can kinda copy it 1 out of 100 tries.