Agreed. NVIDIA could be fine with a Pascal refresh.
I'm sure AMD will work on making improvements to Vega, as it did initially flop for gamers. (It has been great for miners, but...), especially since AMD is including Vega architecture in their new APUs and for APUs, they're looking impressive if the expected performance is anything to be believed.
But until AMD improves their high end, NVIDIA could totally do another Pascal refresh.
I think Vega 56 is a decent card if you can find it at MSRP, but they failed to have anything that competes with the 1080 Ti :(