Food for thought- AMD brought out low level access in the form of Mantle before dx12 was even announced (which is still not here yet) at no extra cost for their own user base (in contrast to Nvidia charging their user base for Gsync), and they were getting stick for "fragmenting the market"
If I'm not mistaken, the "freesync" monitor is basically just "generic adaptive sync monitor" in nature in terms of implementation, which mean Nvidia card should be able to support it as well, provided IF Nvidia would make their graphic card support it. If that is the case, I really think Nvidia would be better off supporting adaptive sync alongside Gsync as well, as it would at least give existing AMD users with "freesync (adaptive sync) monitor" to considering switching to the green team without the penality of losing the sync feature. Not supporting adaptive sync, it would mean Nvidia would be rejecting future customs from anyone that owns a freesync monitor, so they'd just be shooting themself in the foot.
But then again, they would probably put the potential profit on Gsync by denying Nvidia option on adaptive sync vs the potential profit from AMD users and freesync monitor owners that might switch camp in the future on the scale.
Given time it seems inevitable that they will support adaptive-sync, especially if Intel take it up in any way. But they are pretty intent on creating their ecosystem. So I expect they will hold off until they can differentiate G-sync, and since it is propriety they will be able to control minimum screen specs and make feature additions and changes more readily.