[devishard]

> In a world without copyright, large corporations would have free rein to slurp up all the world's public software, extend it however they like, create whatever remaining functionality they needed, and publish none of it. None of what the GPL's copyleft attempts to accomplish would be possible.

That's only true of a world without copyright where no other system of rules around software existed. It's trivial to propose a set of rules that requires code sharing without copyright. In fact, such a set of rules would probably be much simpler than the GPL. Copyleft is really a convoluted hack on top of copyright.