You're basing your claim on speculation. For all we know, the iPhone 6s / iPhone SE and iPhone 7 will be dropped at the same time, just as the iPhone 5s and iPhone 6 were. And the iPad Air 2 has been given six years of OS support thanks to iOS 13, which could indicate that iOS 12 was the start of a new support strategy, not just a discrepancy.
You don't know that, it's also speculation. What makes you think that?, i'm curious to know.
The Reason why the 5s (2013) and 6 / 6 Plus (2014) were dropped at the same time with iOS 13, was the simple fact that they shared the same old incapable hardware. Both the 5s and 6 / 6 Plus had only 1 GB of RAM.
Now, iPad mini 4 and iPad Air 2 get iOS 13 aswell (iPadOS is just marketing), and they have the same old A8 chip aswell, the Air 2 has the A8X and the mini 4 the regular A8. But both of these devices have 2 GBs of RAM, while the 6 does not. Which is why those tablets get support for iOS 13, while the phone does not. Therefore i do believe that the A8 chip is able to run iOS 13, but only with 2 GBs of RAM atleast.
My point however is that i am convinced that the 7 will be supported longer than the 6s, even if it's only by two years. 6s / SE could be supported by iOS 14, or not, i don't know. All the A10 devices have atleast 2 GBs of RAM, some have 3 GBs, and the A10 is also a bit faster than the A9.
Also Apple still uses this chip in newly released products today. Like with the in April 2019 released iPod touch 7th gen or, more important, the 10.2" iPad 7th gen (September 2019) which are both devices that were upgraded with the same old A10 chip just this year. The A10 is also used in the 6th gen iPad from March 2018, just with 2 GBs of RAM. All of these devices are 1 - 2 years old now and pretty popular (except for the iPod maybe).
Let's just imagine Apple drops support for the 6s and 7 at the same time - let's say - September 2021 when iOS 15 is due. The 10.2" iPad would only be two years old at this time and already be unsupported. If Apple drops support for the 6s/7 with iOS 14, that tablet will only be one year old and be unsupported. I can't really see that coming. It's also unlikely that they will support the A10 iPads longer than the iPhone 7, because again, same RAM. 2 GBs for the 7, 3 GBs for the 7 Plus. 2 GBs for the iPad 6th gen, and 3 GBs for the 7th gen.
I think that the 6s (and SE) will become unsupported either with the release of iOS 14 or iOS 15. A10 devices should be supported longer than that.