It wouldn't be possible to bind anyone to the contract. With copyright, anyone who wants to release a modified version of the software needs permission from the original author, and thus, has to agree to the contract in the license agreement. Without copyright, anyone could just do what they want without agreeing to any contract.
Are you sure about that? I'm fairly sure contracts exist today that are both completely unrelated to copyright and absolutely enforceable. I just sort of assumed they could be applied to what we consider copyrightable works if copyright didn't exist.
I understand that permission of the original author wouldn't be needed for anything once the software is obtained if one didn't agree to any binding conditions when obtaining it, but at least the first person to obtain it must obtain it from the developer, and that could be tied to a binding, enforceable contract to which one must agree to obtain the software. Let me explain with an example:
Bob is a developer, and he has created the amazing software "Foo 1.0". Now, Alice wants to use that software, but since it's new and nobody has obtained it yet, only Bob can give her a copy. And so, she contacts Bob, who tells her that if she wants a copy of "Foo 1.0", she must sign a contract. So Alice agrees, signs that contract, and gets the software. Now, I think that this would be reasonable and possible. And then, depending on the specifics of that contract (which I'd say is binding and enforceable), Alice may or may not be able to do certain things with the software she obtained, even if there's no copyright. Is my reasoning correct, or did I miss something important that makes it flawed?
Of course modified versions wouldn't be subject to the conditions of the original contract, but then again said contract could forbid the creation of modified versions, and/or publishing the source code.