I can see Apple supporting IPhone 5S next year. Apple usually give one more year of OS support for S Iphones models. It started with the 3GS, 4S and maybe the 5S will continue that pattern too. However , the IPhone 4S didn't end up well on IOS 9...
I think that IPhone 5S will hold up well IOS 11. But , if the IPhone 5s get iOS 12 in 2018, it will problably be like an iPhone 4S on iOS 9. The A7 processor has 1GB of Ram, 1.3ghz , 64 bit . It is enough for this year , but will it be enough to run well IOS 12? That's the question.
Apple dropped IPad 2 ,IPhone 4S from the line up before iOS 8 was released. And these devices got iOS 8 and 9.
IPad mini 2 (A7) was removed from the iPad line up this year . So, based on history, A7 devices will get 11,12.
The 6S will be able to run iOS 15 wow!!