8700k is an "end of the line" CPU in terms of motherboard support. Also AMD Ryzen CPUs give you much better bang for buck.
AMD has committed to the AM4 socket they're using until at least 2020, so if you buy a Ryzen processor and motherboard today, you should be able to use that motherboard with Ryzen class processors until 2020.
Good to know AMD is doing that! Getting new motherboards is annoying.
Intel switched to the AMD model starting with the i3/i5/i7 series, now moving to an integrated MMU themselves.