I'll let someone else prove that first.
But as cheap as I am, I would still pay the extra to get to the i5-8600K at 5GHz, and when someone did a poll, I was in the extreme minority for choosing the 8600K when everyone was picking the even more expensive 8700K.
On Newegg, the 8600k/8700K are both sold out. The i5-8400 is in stock.
I do not think your necessarily wrong. One of the two chips SHOULD cannibalize some of the sales of the other. Either the 8600k cannibalizes 8700k, or the 8700k cannibalizes some of the 8600k. The purpose of these two chips is either maximum game performance or all around chip. The only issue with an all around chip is you need to consider Company workloads if your a small business of a few persons/friends, who also game. Would it be better to buy a large number of all around chips say 8 of the 8700ks, or would it be better to buy 3-4 8600ks for direct development, and make a large 1-2 rack renderfarm with the r7 1700? This is of course assuming your buying ALL new(many of course will only update 1-2 units per year, which further complicates development).
Overall some might choose A and some might choose B. However, there might be a greater preference to one side, which should become apparent once initial purchases die down and coffee lake production ramps up (not to mention limited purchases until 2018 delaying the ability to get read on the situation).
8100, 8350k, and 8400 are the more interesting parts for coffeelake imo. 8700k OCed should not be significantly faster in ST or MT than a Well OC haswell 6core or OC broadwell (10%) or in rarer cases for those who OCed all aspects including the mesh of skylake-x 6 core.