What's stopping people from doing that now? The moment you sell something of any interest, you can be sure to find a copy of it free on the internet somewhere.
That's the key difference.
The fact that it's illegal limits large scale commercial exploitation, leaving enough room for legitimate distribution that pays the author.
With no copyright whatsoever, there would be a well-organized company with a better marketing department selling their version of your work, taking all the profit, and making it impossible to actually sell it yourself.
That's exactly what the AI companies are doing right now. Except they're not even selling your work in any kind of recognisable form.