As Computers advance, hardware becomes older and as technologies progress, that old hardware becomes outdated, unable to to complete the simple tasks of the new technological age.
People will get the best stuff right now so 6 years down the line, it's as if you just bought a consumer product and it will still work great.
If you buy 8700k right now with a 1080 TI you will not even think about upgrading it for at least 5 years, hardware is becoming very powerful and no game company is going to spend half a decade developing the best graphical game ever when they could just get it out within 2. The only technology that is advancing is VR and that requires high-end stuff right now.
We have really reached a point where upgrading every other year is unnecessary. It is likely you would spend as much money now than later on upgrades.
Edit: Get a 1080 TI and 8700k, 16gb ram is fine but if you're doing more than a normal person then by all means go 32gb.