That's probably because you were trying to come up with a system this is more restrictive than the traditional physical media system (where you could share or sell a single physical item), but with concessions.

There is a very simple and obvious solution - internet connection required on activation or deactivation (for trade). I have not read a single good argument against this system anywhere.

I dont think you understood what i said. The codes are transparent if someone is connected to the Internet. The reason I added them is to add functionality. Also, I would suggest a pure digital copy shuld probably have more permissive digital lending features as a selling point, but there is no reason not to add them for a "loaded" game.