As long as they don't screw up iOS updates I think us 6s owners should be sweet for at least the next 3 years. I didn't upgrade until my 5 year old iPhone 4 started becoming extremely slow at the end of last year. Had to change the battery back in 2013 but it still worked nicely until I updated to iOS 7 but I guess the 4 was outdated when it was announced (higher resolution retina display + same GPU as the 3gs). Seeing as most games and apps are being developed for the 5 and 5s now, I'm eagerly awaiting for what game developers can do with the 6s (Eisenhorn is a solid example of the A9(X)'s graphical power).
iPhone 4's CPU was OK for its time, which remember was 2010. And it remained a decent performer until it got iOS 7, back in 2013. I do agree it was probably the CPU that was a problem to a large extent though, since the 512 MB memory was the same as the iPad 2 and iPhone 4S, which have 2X or more CPU power and which did well through iOS 8. It was really only iOS 9 in 2015 that brought those to a crawl.
I upgraded my iPhone 4 to iOS 7 in 2013 but felt it slowed down the phone too much and thus felt the need to upgrade. However, even if I had been on iOS 6, I would not have been able to last until last year, because of the slow speed and because of the fact a lot of apps stopped supporting iOS 6. Furthermore, surfing on the iPhone 4 was slow for rendering unless you only used light mobile sites. In fact with regards to the OS version, you can't even run some of the latest versions of the apps now unless you have iOS 8 or even iOS 9. Thus, in 2013, I upgraded to the 5S.
On the 5S, I've noticed that there is significant lagginess in Safari now, something I don't notice with my wife's iPhone 6s (A9, 2 GB) or my iPad Air 2 (A8X, 2 GB). So, it's time to upgrade again, hence my order of an A10 Fusion iPhone.
People say that we have too much compute power in our phones, overkill with what we need to do with them. While that may be partially true when the phones come out, that becomes false in as little as 2-3 years, so if you keep your phones a long time like we in our household do, it makes sense to get the latest model.
Actually what some of my friends do is buy a new phone every year and sell the one year-old phones for maybe 65% of what they paid for it. That actually makes sense, because you're always at the top of the heap and you only end up spending a third of the cost of the phone each year, but it's a PITA to sell phones on the used market so we haven't bothered much with doing that.
tl;dr:
Despite all the statements that today's phones are overpowered for what we do on phones, I think flagship phones operate really well for 2 years, and just reasonably well for 3 years. After that, all bets are off. If you keep your phones more than 18 months, it makes sense to want high CPU power and lots of RAM, even though it may seem like overkill at the time of purchase.