They would love to go after the 1080 Ti or even Titan Xp but you can only use what is available in the tank.
AMD need to offer better value for money than you get with the GTX 1080 and all will be fine, if they don't people won't buy Vega just because it is expensive and made by AMD.
Under normal circumstances you are correct but with the predicted hashrates miners will take all the stock fast AMD could charge more and still sell these well. and Vega 56 will be like hens teeth.