Don't get me wrong this is a good thing but in my experience at about 3 years you are looking at roughly 1000 charging cycles and so the battery will barely last 12 hours. Add into that knocks and bumps and scratches and they're basically junk by then. Even if you could replace the battery, the physical wear and tear is quite substantial I find (they're tools after all)
I suppose if you have been ultra cautious and basically leave your phone at home on the charger 90% of the time and keep it wrapped up in some huge case then it is conceivable that you might approach 8 years... But then if you are like this barely even use the phone, why not just get a feature phone?