Lets imagine that we create a third way to link software. There is static linking, dynamic linking, and now we got this imagined third thing called adaptive linking. In this made up linker, it magically combines the different parts and uses neural networks (ie, more magic) to figure out how to make the two parts communicate without a API.

Okey, in this fantasy world, do we still have the concept of derivative work? I would say Yes. Would thus GPL still apply? Yes. Is there an API? No.

GPL is not based on the technology of linking. As a software license, it used the concept of derivative work, distribution and other concept in copyright law to define the scope. LGPL in contrast do specify linking technology, but that is only an additional permission which would need to be modify if it were to function with new linking technologies.

If you created a drop-in replacement for readline, the argument that the complete work is readline + some code get weaker. However, if the police finds some emails on your drive that states your intention to combine readline + some code, and then go to copy readline by reimplemented the software in a identical (copy'ish) way, then you might very well be infringing on the right of the readline authors. It depend what a judge/jury think your intentions was.