You are arguing that future proofing is fine but you are buying the highest of high end,dual GPU's to do so. Of course that will last you a while but 99% of people just cant afford to do so. Buying a single card now,mid range at that,to future proof just wont work.
You really think 4k wont be "the norm" anytime soon? Even the consoles will run games in that resolution by next year! Obviousy there are people running 720p monitors now but add the consoles upgrade in hardware to cheaper panels being released all the time and it will much more commonplace
In terms of a card "lasting", are we talking of how long they can run at a playable resolution and frame rate, or weather they can run period?
For example, I've seen people playing doom on the GTX 470, and it's playable.
Hell, the current consoles have doom, and the Xbox one's PC GPU equivalent would be something like the 7850!
To be honest, it's tempting to get something like the GTX 950. It's very cheap, it works, it's better than current consoles which are said to be supported for a good couple of years with new games, even with the new ones releasing.
That way it's hardly a big deal to upgrade later when we have more information as to how things are panning out next year and beyond (more focus on 4K, more info on Vulkan/DX12 implementation, new card architectures etc)
Just a thought. I wouldn't expect anyone but someone coming from a console like me to be happy with that solution, but for me, as someone who's never even had a 1080p system and doesn't know what 1440p and 4K looks like, it might be an idea.