> The problem is that the OS upgrades invariably slow down older phones, so even if you're perfectly happy with your iPhone to begin with, it starts to act slow as it gets the newer OS's.
This is a truism and from my experience it rings false. I ran an iPhone 5 for four years without feeling degraded.
FWIW, as background, I'm an ex-overclocking PC enthusiast and I consider myself very sensitive to any sorts of performance lag.