I really don't understand why on earth Apple would wait until Fall at this point - that's far too long. I'd be concerned very highly that they will have massive bleed of their core user base. At this point, I would buy today any of the following, if offered with MacOS:
At this point, I can only think of two reasons:
1) Fixing the keyboard, and/or improving the touch bar
2) Waiting for AMD's Vega Mobile chips to bring 1050/1050Ti-tier performance without having to compromise on size/poweri/heat limits.
But your last part - "if offered with MacOS" - is precisely why they can afford to wait a bit longer.
All of the above are available with equal or better specs to MBP, at equal or (often much) less money and offer better customization and/or more choices. They all have off-the-shelf components which apple could put into a 2017 chassis right now... and I have to think that a large amount of people (if this thread is any evidence) would massively flock to and purchase sharpish.
Everything you say here is correct, except for the part about Apple being able to put these into a chassis.
The 2017 chassis doesn't have the means to dissipate something with an 80-90 watt TDP (which the 1070 Max-Q has), and by going with charging over TB3, Apple has capped the overall potential power consumption of the laptop at ~100 watts (by virtue of the limits of the TB3 spec).
But the Apple tax funds more than just the laptop specs - build quality, and the Apple store network are the two major ones that come to mind (being able to walk into a store and just get your laptop fixed can be an immense convenience).
I just don't get it. If neither their laptop nor their desktop are modernized at WWDC to processors / cards that have now been out for months, am going to have a hard time not giving up on them and just buying one of the above and/or just building a Windows PC for $600 less at twice the specs.
Laptops with Coffee Lake have not been out all that long. Intel announced the mobile lineup two months ago, and laptops with those processors are just now making their way on the market.
GPUs from team green have been out a while, yeah, but Apple isn't going to be using those, so I don't think it really makes that much of a difference.