Check this post, I collected some market data in October:
https://forums.overclockers.co.uk/showthread.php?p=30092340
Keep in mind, FreeSync is more appealing not only for customers, but also for manufacturers; The nVidia-tax is quite a tangible part of the overall manufacturing costs for G-Sync monitors. For manufacturers, the profit margin decreases, and thus their percentual return-on-investment suffers.
Whereas with FreeSync (/Adaptive-Sync, which FreeSync is based on), you can implement it to your monitors for next to nothing, so there's not much sense to NOT do it. That's also one of the main reasons why most manufacturers offer it more often than G-Sync.
As for customers, it's naturally more compelling to purchase a monitor that costs £100-200 less, but offers the same features. The same price saving could be transferred to purchasing a more powerful GPU, which in case of FreeSync, is by default AMD's.
Also, because the price difference versus a regular monitor is relatively small, there's no compelling reason NOT to purchase a FreeSync monitor, if the other alternative is one without a variable refresh rate, at all. And because of this, more manufacturers will have to implement it, otherwise they will become the "runner-up" in comparisons.
So, if you're an AMD GPU owner, there's no reason to purchase a G-Sync monitor. But if you're an nVidia GPU owner, and you don't want to pay extra for the G-Sync, then there's no logical reason NOT to buy a FreeSync monitor, instead. The FreeSync option will even remain for a potential future AMD GPU purchase, whereas you can for the time being use it as a regular monitor, in any case.
As such, nVidia is actually taking a huge risk, when they refuse to even support Adaptive-Sync. Currently FreeSync monitors and AMD GPUs are perfect complementary products. nVidia's reluctance, which drives more customers and thus manufacturers towards FreeSync, can also drive customers towards AMD's GPUs, as well.
I would reckon that nVidia has smart people in their marketing department, who should be aware of this. So the bigger question is: do they have something up their sleeve, that the market is yet unaware of? Or maybe they're just hanging on to a foolish hope, who knows... But the situation surely ain't looking good for G-Sync, at the moment.
Even Intel has already given their support for Adaptive-Sync, so my take on the situation is that nVidia will eventually just have to swallow their pride and at least start supporting Adaptive-Sync.