I'm about 99.9% certain, based on the language on Apple's website, that the W1 chip is _only_ inside things like the AirPods, or the new Beats 3s. It's not a co-processor for BT, nor is it a successor. It's the SOC for the headphones that handles not just the pairing, but also dealing with all the sensors inside the AirPods for seamless playback over a mono or stereo connection. My understanding is that if you only have 1 AirPod in, it won't just give you that side of the stereo playback, it pipes both to the 1 Pod. Then, when you put the other one into your ear, it returns it to true stereo playback.
Also, look at
Apple's official device compatibility list, there are some old devices on there that definitely wouldn't have a W1 chip. The cross talk pairing is largely handled via iCloud, and requires the latest OSes.
The rest of your points regarding the value of a a 7+ compared to a 6s+ are valid and correct. I'd add that the 7 series also has stereo speakers, (highly probable) low level internal improvements to the design (think the internal reinforcements from 6 to 6s), a little more battery life, and a better display.