I paid $849 for an iPhone 6s 128GB in 2015. Not only is it still getting OS updates over 3 years later, according to benchmarks, it is faster than any Android Phone in single core performance that was released in 2018 - including the flagship Galaxy S9.

Since the 2013 iPhone 5s is still getting updates, I suspect that the 6s will still get updates at least through 2020.

But right now, you can get an iPhone 7 for $449. Again, if history is any guide, it should get OS updates through 2021.