My iPhone 6 is from 2014, next year it'll still run a supported OS that seems to be about as snappy as the previous version. Assuming a yearly iOS release that's 4 years next year (assuming they stop supporting iPhone 6 in iOS 12).

I think prices have stayed pretty much the same or become lower over the years. You can get a 64GB iPhone 8 this year for $699, vs $749 for the iPhone 6 in 2014. I think the iPhone 8 is still a pretty "premium" experience, even if Apple now has a more fancy and more expensive model.