I wouldn't worry about it too much. These things have been repeated over and over. There's no point worrying about the future, just buy whatever you fancy and enjoy it. You can't keep planning for what happens in the next 2,3,4 years, just buy something good enough so you actually enjoy it instead of watching a stuttering mess.
1070 is a beast of a card and it will serve you well for a several years whatever happens on the API scene. It has the raw power, the memory and efficiency to not have to worry about anything else. There's no point in waiting for some insignificant gains to be had in 3 years from now, with a 1070 you'll enjoy the power you need right now in all the games not just the one or two where an AMD card might shine for a bit.
Look at the numbers and don't get fooled into believing that Doom or Vulkan will guarantee anything. I played Doom myself and while I enjoyed it, that didn't last more than a week or so. You don't buy a card for a week, you buy it to get performance in everything you're doing, all the time.
If there's one thing I would recommend is to look at 10-15 games you fancy now, check the numbers, settings, quality etc, consider your budget, your monitor and resolution and buy whatever makes sense for you. Just don't buy based on 1 or 2 games and one API which may or not be used by anyone else.