Are you sure? This was my reasoning when I bought a Nexus S in early 2011 just after it was released. It came with Gingerbread. Later, at the end of the year, they released the ICS update, and then subsequently was blocked. One of my friends got it, but I never got it (until I downloaded the ROM from somewhere and manually updated). The official line was that this was due to poor battery backup, but I felt the battery backup time improved after the update. Eventually I ended up selling it for 100$ and buying a iPhone 4s.
Clearly, all hardware manufacturers have an incentive to not give updates in order to compel people to buy their latest devices. It's just that Apple plays a slightly different game by giving updates, which make your device slower and slower with every upgrade until about 18-24 months later, you just fork out cash to buy their latest device.