This may not be that much different than socket 775. Socket 775 had a long life, but there were many new chipsets for it. Of course, you could put a clunky Pentium 4 in the newest boards and it would still work... it was the newer chips that often needed the support. Kaby Lake not working in the new Coffee Lake boards makes no sense. Sure they can say power delivery and whatnot, but how is it then that the same board capable of running the then flagship QX9650 could also run a much older Pentium 4?
@Bill_Bright I think the general point here, the point you seem to be refuting, is that there's no logical reason why the Kaby Lake shouldn't be compatible with the new boards, or Coffee Lake shouldn't be compatible with the old ones. While a new board with the latest features is always nice, I think even the majority of us here with a Kaby Lake chip, or even the older Skylake chip, who would be interested in upgrading to Coffee Lake wouldn't care as much about the updated features on the motherboard as we would just having a better processor, but for one reason or another, Intel says we have to buy the motherboard too, when the old one is still likely more than capable. It's very likely Intel could have made it compatible, but they chose not to, which kinda ticks off some people who would be interested in the better chip, but not so much the better board. It doesn't matter much to me personally because I rarely upgrade my system, so I, like you, would likely be changing my motherboard anyways when the time does come, because I've had my system for so long that I can't just drop a new chip in. I'm too many generations behind for that, and there are some things I don't care for about my motherboard anyway... but I still see this as a bit of a dick move by Intel.