Realistically speaking, both T-Mobile and Apple stand to gain if they can get you to upgrade your hardware. T-Mobile most certainly gets a cut from the sale of new phones.
It's sad because the iPhone 4S (which I have had now for 3 years) is an excellent piece of hardware. I was worried that it might get a bit slower when upgrading to iOS 7 and now iOS 8 but I have seen little to no degradation in performance.
As a company, the catch 22 of making great hardware is that it doesn't need to be replaced for quite a while. So it seems like companies in this business try to walk the line of making hardware that is good but not so great that you don't need to keep buying a new one every couple years. And to help the practical among us, they release useful features that only run on new devices so we can justify it to ourselves.