You say GNU would not exist, which is true in it’s current form. But that doesn’t mean we wouldn’t have had a Unix-like (although not strictly compatible) free system.

It’s also true that Safari couldn’t have used XMLHttpRequest unless Microsoft had licensed it. But that might have sped up the incorporation of an open standard alternative.

It would definitely have created a different history to the one we have now if APIs were assumed to be copyrighted, but it’s not reasonable to just assume that everything that has been based on them would not have been built some other way.