Asymco estimates how many iPhones will be upgraded in next year.
asymco.com
asymco.com
Despite what the blogosphere seems to emanate, the existence of faster devices doesn't render the older ones unusable.
But I agree with your main point. I switched to an Android device that had even less processing power than the 3G and I don't really see much need to buy a new phone until this one falls apart.
I think "until it falls apart" is a healthy approach to ownership. I plan to do the same with my MBP. Part of the reason for buying quality items of all types is that they happily last.