The problem is the 1070 was out in June 10, 2016. It took AMD 14 months and counting to even have an answer for the high-end market. They allowed Nvidia to hold the high-end market completely to themselves since the 290X. Fury X/Non-x weren't exactly the most popular cards either. This is the problem with AMD. It's not that they can't make fast and great things. It's that they take forever to do it and constantly give free money to their competitors.
Also, Vega 56 numbers so far are rumors. I bloody hope it's good because if it comes after 14 months after the 1070, comes with a higher TDP and performs the same. Then well, Idk. Just kinda silly.
I think you forget some of the reasons why people go to AMD,
That can be applied to the RX 4XX/and 5XX series.
It's just people go to AMD for, wait for it,
FreeSync! Oh also not to mention DirectX 12/Vulcan advantage (since they seem to take those into consideration). And crossfire
I mean, RX Vega FE is a "unique thing" if you will.
Since Titan Xp accerelates at gaming, while is still "decent" for creation usage, compared to FE which is completely the opposite, it is way better than XP with content creation while being "meh" at gaming (for christ's sake it's between 1070 and 1080)
Yet the RX 4XX/5XX and the Vega all have one thing in common as it seems, Value/Performance.
They usually have been the best for your buck when compared to GTX 1060 (before the mining market came in)
But yeah, who knows.
I mean, assuming from the unknown source is "true" The 1070 will most likely have a price drop.