You get a couple of years of security updates, and new software starts to use the newest features to the fullest in the meantime, so when you take the plunge, it's always a bigger, more worthy jump.
You can safely leave the device behind then, to use as an emergency phone, if the need arises.
So, 1-2 years is not that crucial, but after that you really start to feel the gap. Same for Macs.
