Well I bought my phone last year and there is already dwindling support for it (no OS updates, just extra HTC crap), not to mention that the same model is still on sale as new now.

Many people buy smartphones on a 18 month - 2 year contract and often will wait until the price has dropped a bit to buy it. So I think 3-4 year old phones should still be well supported if they want to gain people's trust.