the thing is, for those people who have had phones for years on end, how often did you receive software upgrades that did more than bug fixes? apples iphone OS upgrades are not just fixing bugs and tweaking little things, they are adding an entirely new layer of functionality. then theres the fact that these phones are being used in a completely different way to the phones you probably used to keep for years on end. its not just a phone/text/email device, it's a computer in your pocket, and like with computers, an OS has more and more demanding requirements as they add more and more features and eye candy.
like i said above, the 1st gen is clearly capable, but they have to leave it out at some point, 3 years from launch (or 2 years after its death) seems a good time to me, that way even if you bought one on the last day of its life cycle you're still safe (if you bought it and used it as apple intended... on and through each countries respective provider) because your contract will have expired and you can renew it for another subbed iphone. apple are not going to take in to consideration the people that bought it second hand or unlocked it to put it on another network.
in fact thinking about it, i'd be surprised if the 3g does lose support next year, but instead loses it in 2012, since that will mark 2 years since the end of its life cycle.
as a disclaimer, this is clearly guess work. i have no inside knowledge. it just makes sense to me because from what i understand the average contract length in Europe and the US is between 18 and 24 months, please correct me if i'm wrong about that.