Two years of major-release support, and three years of security support: http://www.androidpolice.com/2015/08/05/google-announces-new...
(I am no iOS fanboy, but honestly I'm sick of the android expectation of getting a new phone every 2 years, and they seem to be the only ones who are even pretending to give a shit with their old phone support. They still don't give as many shits as I'd like, but I cannot find their better).
If I wasn't so wed to Android the right thing to do would be to buy whatever iPhone was new at the time and ride it down until they stopped providing updates. Especially because (anecdotally, of course) iPhone battery life seems to last much better over time than Android, so not only will I continue to receive updates for longer, but it will actually be a usable phone for longer.
https://support.apple.com/en-us/HT201222
I believe your overall point remains valid, though. The most recent hardware that can't update to iOS 9 is the iPhone 4, which shipped five years before iOS 9, and was last sold 3 years ago.