I agree with the 1080ti suggestion. Not all games will work properly with SLI, so in those you might just be getting the performance of a single 1070.
Your resolution will strongly effect how long the performance would be good for. If you're planning to play games on a 4k monitor at their native resolution, it might not be that long before you will have to lower settings to get good performance at that resolution, since it's like rendering to four 1080p monitors at once.
On the other hand, if you were only planning on using a 1080p, 60Hz monitor, those graphics cards would probably be complete overkill, and you could probably go with a graphics card setup that's not quite so high end and not notice any difference in performance in the near-future, then upgrade to a new GPU a few years down the line.