Yet you haven't explained why an existing KBL won't work with Z370
We've gone over this. There is no point in Kaby Lake working with Z370. No one is just upgrading their motherboard, so it doesn't really matter. Furthermore, that isn't what we are discussing. We are discussing Coffee Lake working on Z170/270. That was your original statement, that is what I responded to, that is what we are discussing. Because no one gives a rats ass about Kaby Lake working on Z370.
or why does an i5 8400 sell with just Z370 to pair it with
Well that is because Coffee Lake is a rushed launch. The other chipsets, and lower end processors, aren't ready yet. Q1 of 2018 sees the release of H310, B360, Q360, Q370, and H370. Q1 or 2018 was originally supposed to be the launch for the entire Coffee Lake line, but Intel pushed the enthusiast portion up to Q4 2017 to compete with AMD's higher core count Ryzen AM4 processors during the holiday season.
My theory ~ they've shut out existing KBL owners from upgrading to a hex core, & possibly octa core in the future, deliberately!
And your theory is wrong.
I've already explained this, let me repeat ~ was z370 a necessity?
Z370 was nothing more than a moniker to separate compatibility. It is there to tell you that the motherboard you are buying is compatible with Coffee Lake. They didn't change the number of pins, so without renaming the socket, the next option is to rename the chipset. And even if they did rename the socket to something like 1151-V2, they would have incremented to chipset number as well just to stick with tradition.
The change in pin function on the socket was necessary.
Possibly, but then how do you explain KBL being shut out of working on Z370, maybe Dave can answer that as well?
There is no reason for Z370 to support Kaby Lake. Do you really know a lot of people rushing out to upgrade their motherboard and keep their old processor? In your fantasy world is that something that is in high demand? And why would they even bother, Z270 has pretty much the identical feature set to Z370.