That's one way to look at it, but another way is to bring higher level of performance to more reasonable price levels -- in a way AMD brought $650 GTX780-class beating performance to a far more reasonably priced $399 R9 290. Right now,
the cheapest good 1080 is about $629.99+, with many GTX1080 cards hovering in the $650-700 USD range. Even if AMD has some Vega chip that only matches GTX1080, but the card is priced at $499-549, that would be a huge win for consumers because we'd get a cheaper card with superior DX12 performance.
We'll have to see how the future unfolds but AMD has had extremely competitive offerings during HD4000, 5000, 6000, 7000 and R9 290/390 generations. It's only the Fury generation that can be considered the first major flop on their end. The last 2 generations, for anyone who keeps their GPUs longer than 18 months, AMD clearly won not only on price/performance but also on high-end performance:
HD7970/Ghz > 680
R9 280X > 770
R9 290 > 780
R9 290X > 780Ti
R9 295X2 > 780Ti SLI
Today, the high-end Hawaii R9 290X/390X trades blows with GTX980 and sometimes approaches 980Ti in DX12 games where 780Ti is beaten by an R9 290. Ouch. That's another point many are downright dismissing -- Vega may launch later but at least it will be a proper DX12 architecture whose GPUs can be used for the next 3-4 years.
I don't doubt that NV will retain the GPU performance crown but for 99% of PC gamers not on 1440 144Hz, 3440x1440 or 4K monitors, these GPUs are largely irrelevant if NV charges $800 USD for 1080Ti and $1200 for Titan XP.
If AMD just undercuts GTX1070 by $50 and 1080 by $100, those NV cards won't be worth purchasing due to lack of a proper DX12 hardware/architecture; and NV's lackluster driver support for GTX600/700 series. Since modern GPU generations last roughly 2 years, AMD will have at least a year to sell high-end cards. Ironically, as we've seen during HD4890, 6970, 7970/7970Ghz, R9 290X and Fury generations, AMD's high-end cards are rarely purchased by high-end consumers. Therefore, even if Vega are great GPUs, they will hardly be popular among enthusiasts who as you said either already purchased Pascal, are waiting to replace 980Ti with 1080Ti, and/or are by now locked into G-Sync anyway.
Where AMD needs to concentrate with new GPUs is for newcomers to PC gamers who don't have inherent biases with GPU purchases. I would guest-estimate that a lot of people on these boards are veteran PC gamers, at least in their early or mid-30s
and older. There are also millions of young PC gamers between 16-28 who are either interested in 4K gaming/VR, may just be may be entering PC gaming for the first time or building their 2nd PC. That's where Vega comes in. By 2017, the current gen consoles will start to look very aged and usually towards the end of the console generation, a lot of gamers decide to jump ship to PC (hence why Sony is trying to prevent that with PS4 Pro).
Depends on a gamer's backlog and current pricing in their country. Newegg Canada sells the cheapest decent AIB 1080 for $925 CAD after taxes. We are talking almost $1,000 CAD for an upper-mid-range Pascal chip. Not exactly enticing to purchase considering that today a $200 USD GTX1060 matches or beats a barely 2-year-old $550 USD 980. The era of 'needing' $1,000 CAD GPUs is pretty much over. PC gaming today is nothing like the old days where if you had a high-end card, your PC gaming experience was dramatically better. Back in the days it was a difference between running the latest games and not. Today, a basic $250 GPU such as RX 480/1060 is going to be a great gaming GPU for the vast majority of PC gamers running 1080p 60 fps or lower.
Consumer Volta has never been on NV's roadmap for 2017 but we continue to read how Volta is coming in 2017. If 1080Ti launches in 2017, why would NV canibalize their gross margins by releasing a $450 Volta card that obsoletes the $800+ 1080Ti?
680 March 2012 -> 980 Sept 2014 = 2.5 years
980 Sept 2014 -> 1080 May 2016 = but with real world retail availability approaching close to 2 years
If NV has 1080 Volta successors next year, it's unlikely to launch before November 2017 at the earliest, but chances are NV will wait until 2018 to launch Volta because they can continue to milk Pascal for all its worth. NV can easily drop the price of GTX1080 to $499-549 and not even sweat it.
The other thing is some of the most anticipated titles released in 2014-2016 run well on a $250 RX 480/1060 6GB GPU at 1080p and lower resolution. That means the dGPU market for $500-1500 GPUs is extremely small. What AMD really is a refreshed RX 485 that convincingly beats the 1060 6GB, a more solid card between RX 460 and 470, shift RX480 8GB to $199 price level and introduce a good GTX1070 competitor. Anything above that quickly hits diminishing returns (i.e., because NV loyal buyers won't buy NV, while the fraction of brand agnostic/AMD consumers who are willing to spend $500 USD+ on a card isn't that large). Probably one reason AMD is even making Vega is so that they can shrink all those chips later to 7nm as next generation's mid-range cards and for brand image. If AMD has faster high-end cards, it will drive the sales of lower end RX 400 series.