And who do you think buys the RX 470? or RX 480? Its gamers also.
2x RX 480 is faster than 1x 1080.
If 2 of the 480's are cheaper and faster than a 1080 why in the world would gamers overpay for a 1070/1080
The Nvidia's will tank in price also.
Plus its easier and faster selling something that's cheaper.
I remember how hard it was selling an R9 280x or R9 290 during the Litecoin crash of 2014. Nobody in the second hand market could afford to pay $200-$300 for a used GPU. But the R9 270X sold super fast for $100.
The Nvidia 980 launched at the end of 2014 for like $550, right now a year later its going for about $200 on eBay. How is that 80% off retail? More like 70% off retail.
IMO, a few things wrong in your analysis.
1) 2x 480 might *mine* faster than a single 1080, but for gaming the 1080 (or even a 1070) would win hands down at most tests - using crossfire doesnt offer a 200% performance, its more like 150% the speed of a single card
http://www.tweaktown.com/articles/7770/amd-radeon-rx-480-crossfire-beating-geforce-gtx-1080-4k/index9.html (tldr: CF RX480 perform weaker than the 1080 in most cases, and outperform in some games that are optimized for multiple GPUs)
generally speaking, a single GPU is bound to work great at everything, whereas crossfire (or sli) only help on games that are programmed to benefit.
2) 2x480 costs the same (or slightly more) than a 1070. The 1070 has better power consumption, and as mentioned is better for games that dont have any DX12/crossfire benefits.
3) the 980 launched over TWO years ago (end of 2014, versus its now 2017). not to mention launch pricing is always high, and drops 10-20% within the first few months. the top-of-the-line cards sell at a massive premium too as they are aimed at a hardcore crowd. the best performance/$ is always found in the mid-tier cards (the 270/280, the 970/1070, etc)
and finally, as amph mentioned the 10XX series has done an excellent job of making money since release - the 1070 made almost $20/day at lbry for a week or two. I bought mine a few days into the hype, and its made back ~80% of what i paid so far, and the card still holds ~70% of its original MSRP on kijiji/ebay