Nothing, no one can stop this now. It's like horses being angry at cars.
A lot of that existing content is copyrighted, and demonstrably present in the resulting product; so these companies have an uphill battle _not_ to be stopped by the courts.
These lawsuits will go exactly like in the war against piracy, impossible to win. Cut one head off, ten more appear.
I'm not arguing that generative art will cease to exist, I'm just arguing that the path to legal revenue will involve paying copyright holders; and the real gold in this industry is in licensing training datasets to businesses that just want 'AI Magic' in their product marketing.