Most likely in an attempt to get people to commit to the more expensive platform. IE it's a call that, spend £50-100 more on a more feature mobo, buy this cheaper chip but later on you can upgrade to a 6-10 core cpu. It's the whole, look, more upgrade options, woo.
It's also largely because they have nothing particularly sensible to fight AMD with in the next couple of years. They don't have CPU only in the mainstream, they can't just add a i5 with HT in the mainstream... because it 100% devalues the i7. They don't have high clocked chips on the 2066 platform, that would be what the quad core looks to rectify as well.
Again, most users, even desktop users, don't want a discrete gpu. saying it will become irrelevant because PC users won't be willing to pay for an igpu is frankly just ridiculous, yes they would, almost anyone outside of gaming has no interest in a second card, even though thanks to Intel prices and Intel performance d-gpu is actually cheaper.
As for power, again yes, lots of people are interested in power on desktops.
YOu seem to be doing that thing where you're using your own requirements for a PC and applying them to the entire market. I don't care about power usage when under load, I DO care about power usage when idle. If I want performance that can only be had by 800W under load, who cares, but that doesn't mean when 80% of the time on my computer watching video or browsing the web I want to use 800W.
the massive majority of PC sales aren't to gamers, very few want a discrete GPU given the choice, most would prefer value and form/function over things like where the gpu is located. igpus enable significantly smaller form factors and done right, reduce costs and power usage.
The gpu being half the die of a £360 i7 is a joke, it gives a tiny fraction of the performance of a discrete gpu at ~£180 that is what you're paying for out of that £360 chip. However a £80 Bristol Ridge quad with £40's worth of igpu performs well for the money, not brilliantly but close to what it should be. A £150 quad core zen APU with probably 2-2.5x the performance of the gpu in Bristol Ridge would absolutely give it's money's worth at £75's worth of transistors.
Then we have other things, the vast majority of enthusiasts don't need igpu... right enthusiasts = gamer... oh wait, no they don't. The vast majority of gamers would choose a d-gpu if given the choice... but the vast majority of gamers don't buy high end cpus, gpus or motherboards. Meaning the kabylake quads on 2066 have zero relevance to them.
You also have no idea if the quad core 2066 Kaby's are going to be any cheaper than the quad core kaby APUs on the mainstream platform.
Why buy a 2066 platform.... that has literally ZERO ability to use the 2066 pins, seeing has a huge number of them are for the other two memory channels? A slimmed down 2066 platform with cheaper mobos.... is the current mainstream platform. Anyone buying a 2066 mobo without any of the features than make the 2066 platform what it was, would be insane and mobo manufacturers aren't going to design an entire range of 2066 motherboards just to fit a couple 2066 Kabylake cpus, because again mental.