Agree fully that it is not iOS related, or even 10 for that matter. I am curious, your 6, with iOS 9, how many cycles? It matters quite a bit before drawing conclusions.
1) Today's Li Ion batteries are used differently than prev generations. The older phones had a fraction of the mAh demand vs current ones, which now exceed the computing power of many PC's years ago. Just the screens place a high demand. It is not just high demand, but you also get mAh spikes, which cause fractures inside the Li Ion materials and internal shorts (Goodenough 2017). J Goodenough, at 94 yrs, is working towards tripling the energy density inside batteries and removing internal shorting. Another issue is when only a few millions phones were made per year, and one top supplier could provide a QC reliable controller (e.g. Texas Instruments), the likelihood of issues was far less. We are now at 500 million mobiles produced a year. Take the Nokia 3200, still used in poor countries. The low demand on its batteries are the reasons why they will not experience weird shutdowns, too low of a demand to damage it quickly. But, pass a 2nd year, even those Nokias are delivering far less life than originally, especially in hot climates.
2) Artificial metering should be there to maximize phone life, not reduce it. Ideally it only kicks in if a battery becomes unstable. The recommendations I provided to Apple have been that instead of metering and causing confusion, best if it identifies irregular amperage and simply notifies she/he that the battery is defective. Am confident it is coming anyway.
3) Have yet to see any evidence of programmed obsolescence, and similar issue occur in non Apple brands as well. There are iPhone 5s out there running iOS 10. Your 6 is nearly x4 more powerful than a 5. However, any phone out there with, for example, an old or marginal battery, will underperform. Amperage changes impact electronic performance. CPU gets less power, it maybe irregular, so does NAND etc and the phone seems to act odd.
What there is plenty of evidence around is that social media fusion demand, and the new HTML 5 based web, are increasing exponentially and exceeding mobile release cycles. Take any of these apps: Facebook, Whattsup, iMessage, Snapchat, Pokemon Go, TV News, and Twitter, and the phone now acts as a fusion platform for 24/7 delivery. Some became very rich with Snapchat's recent IPO, but a year ago many never heard of it. What this means is that your iPhone was designed 3-4 years ago, released two years ago, and running on Just Now high demand processes. Even NAV and Google Maps have changed, it is now navigation, orientation, search, points of interests and traffic conditions in one app. Was not as such 4 years ago, we had to pay for similar features.
Take Internet browsing and Safari- not only has content changed to deliver more video feeds per page, but we now get tons of those active media cookies, inside each page we load. Even on my iPhone. On a PC or my Mac I use two ad blockers. Not so easy on mobile browsers.
In other words, if you change your iPhone 6 battery- through a Genius Bar, do a clean install, and use the phone with above things in mind, it should work very closely to the day you bought it.
Below, J Goodenough, recently making headlines with his new battery projects. We will all see them sooner rather than later.