IIRC the Titan X was basically made to be a benchmark setter. Like the first hypercars. Something that was created just to see what could be done, and when people went "oh wow holy shit, it can do that?" cards started to be made with that sort of technology. So you have the 1080 and the 1070 that can meet or exceed what the Titan X can do and make it readily available to the general public.
I could be totally wrong about the above, but that's what it seemed like to me. Either way, unless you want to do things in resolutions higher than 1080p, the 1070 is probably just fine to upgrade to. It should able to tank pretty much anything you throw at it.