The problem with an 8700k for a new system is the pricing. It sits in between the price of a 3700x and a 9700k, and that's a problem. It needs to be cheaper than a 3700x to make any sense.
An 8700k is basically equivalent to a 9700k for gaming, yes, but the "basically" i.e like a couple of % behind, is enough to make it equivalent to a 3700x for gaming. And a 3700x is faster for everything else, has a more modern chipset platform with better support, and comes with a usable cooler which ofc further highlights the price issue.
If you plan to OC the 8700k to the max then maybe it could be considered, otherwise no.