It would obviously be better if everything were done in the open. But you can't simply say "corporations will corrupt AI with copyright regime X" and compare it to a hypothetical universe "copyright regime Y will result in corporations acting with altruism." Regardless, corporations are going to be rent-seeking parasites trying to maximize their profits at all costs.

The world you're arguing for isn't one where small-time artists are adequately compensated for their work. It's one where Disney and Elsevier collect a bunch of rents and hold back development of tools that would radically improve human well-being. Even in the best case, your copyright suggestion will privilege the giant corporations who have the capability to navigate copyright rules against a bunch of other giant corporations, at the expense of smaller researchers and hobbyists.

I'd be more than happy to sign into something saying "a small artist has a right to prevent training on their work without permission for a decade after it's released." But that is not in the cards, unfortunately.