> Mr. Martin was also paid to support the production of GoT, not just royalties. There is no reason to believe that he wouldn't be called to do the same sort of consulting work on the script, dialogues, visual, etc if the copyright expired.

I find it highly probable that the negotiation for the rights to adapt for the screen included contractual agreements for him to do so to prevent them from hiring someone cheaper who would screw with his intended vision. He had leverage to ensure this outcome. They didn’t pay him a great deal more than they could have paid some unknown name because they are nice and friendly.