I think as long as AMD get the performance and price right for their next new cards, people would be very interested in them; though anyone that got the money and would buy Nvidia only would most likely bought a Titan 2 before that.
As for Nvidia's side, I seriously doubt they would bother to hurry to release mainstream fat Maxwell to counter it, as they would most likely have a Titan 2 out before AMD's flagship card to show off the performance even if it is stuck on 28nm to tease their customer base, and when AMD's (assuming 28nm) flagship comes out as fast/slightly faster than Titan 2, some people will go "meh" still sitting back waiting for the next Nvidia mainstream flagship cards, as they would believe that the performance jump from 28nm Titan 2's to 20nm fat Maxwell for mainstream release would be huge- however the catch is that there will be not confirmation or reliable leaks on whether or not the next 90/90Ti or 90/80Ti (whatever the next mainstream fat Maxwell is called), so people who are hoping for a 20nm fat Maxwell might get a 28nm fat Maxwell shove in their faces...but still end up buying them either way.
Pretty sure for Nvidia, it would be most ideal for them to milk the 28nm 970/980/Titan 2 for as long as they possibly can, before actually release mainstream fat Maxwell, regardless of it being 20nm, or just another 28nm release.