The chart cuts off after 3 years - the point is that the original iPhone was supported with the current version of iOS for 2 years after the last one was sold (minus two days - iOS 4, the first version to drop support, came out June 21, 2010).
On AT&T, at least, most iPhone customers are eligible for an upgrade after 18 months because of the cost of their plan, so everyone was eligible, by a significant margin, for an upgrade before their phone was droppped.