The missing piece of the puzzle is: the goalposts have changed. Apple is increasing the price of iPhone and wants higher ASP.
In 2018, Apple is expecting iPhone 8 owners to move to $999 (5.8") and $1,149 (6.5") models. Not stick with $699 and $799 models. Last year was a transition year so consumers would get used to the $999 price tag.
The 6.1" model is not targeted at iPhone 8 owners. The 6.1" model is targeted at iPhone 6 and 6 Plus owners, which has been Apple's best selling iPhone model ever. Those owners saw no compelling reason to upgrade to iPhone 7 or 8. And the X was too expensive. If you look at it from that perspective, it makes complete sense to leave out 3D Touch and a glass back.
If the 6.1" model included 3D Touch and a glass back, what's the incentive to pay $999 or $1,149 for the premium models? The only remaining differentiator would be dual camera.
There would still be a number of other differentiators besides the camera (OLED screen,
much higher resolution, true edge-to-edge which the 6.1“ might not have, stainless steel frame, much higher battery life, and on top of that some of the new features like the rumored 5G compatible Intel chips might be exclusive to the 5.8“ and 6.5“ models). But you raise a good point, admittedly many people don‘t care that much about many of these features and just want the fancy iPhone X design for the lowest price possible.
On the other hand, leaving out core features on what is price-wise still a mid-tier device would make the lineup seem unnecessarily inconsistent and arbitrary, and it frankly doesn‘t seem like a very Apple-like thing to do, just to arbitrarily push people to the more costly devices. In the past, Apple hasn‘t really been afraid to „cannibalize“ their own products in order to provide the best user experience they can for a given price. The simultaneous iPhone X/iPhone 8/8+ release is the most prominent example of this, but there are many more examples of products which collided with each other but where Apple still went out to make them as good as they can, for example BeatsX and AirPods being released so close to each other, or the iMac Pro vs normal iMac, 12“ MacBooks vs the 13“ entry-level MBPs, and so on.
Also, by that reasoning, why wouldn‘t they have started this trend with the 2017 iPhones already? You mentioned it would be a transition year but really, it would have been relatively easy for Apple to justify having wireless charging only in the iPhone X and not in the 8/8+, much more so than putting that feature into all phones in 2017 but leave it out in what is price-wise its successor. If they want to push previous 8/8+ owners towards the X price range it would be much more reasonable to do that right from the get-go, as opposed to having these features in the 2017 800€ phone but take them out in the ≤800€ phone in 2018.
Also I personally don‘t think Apple expects everyone who owned an 8/8+ or older to move upwards into the 1000+€ range. There are still numerous people who don‘t want to put 1000+€ into a new phone, even less so on a 2-3 year basis, and who previously have been very content with what the 700-1000€ iPhones offered. Upgrading from, say, a 6s to the 6.1“ phone for about the same (or an even lower) price only to find out that a core feature like Force Touch has been removed.
Don‘t get me wrong, I see your point and I‘m afraid you might actually be right and that pushing people to the more expensive phones is Apples reasoning. But it still seems like a rather un-Apple thing to do that would make the lineup unnecessarily confusing and inconsistent. Not to mention that would strongly hurt AirPower sales which is already shaping up to be a niche product (at least if the $200 price tag is true). It would be not very Apple-like to introduce a new AirPower mat but then steering away from it just six months after its release already with at least one of their flagship phones.
