I think the only point we'll see nvidia supporting adaptive sync will be if the majority of monitors (especially cheaper and mid range) out there start supporting it by default and people buy them "by accident" for the other features they offer, and later think, "now that I'm upgrading my gpu, I might go AMD since my monitor supports free sync already". Then, nvidia risk losing market share as people then think "man this sync stuff is cool, I'll stick with AMD".
So it would have to happen organically. I don't plan on buying gsync monitors because I don't know what AMD will bring to the table next year. If the monitor also supported free sync, that would potentially be a different story though.