True to a point, but they don't have the luxury of assuming that you'll buy from them. Not upgrading their devices works against a repeat consumer, really, so you're just as, if not more, likely to help the bottom line of one of their competitors if they leave you in the cold.
And of course there's the march of progress, and the nexus one of one year ago is now outmatched by almost all devices with better screens, dual cameras, coming dual core processors and much better GPUs, etc.
The Nexus One may not match the hardware specs of some of the newer phones, but that doesn't mean it is out of the game. My Nexus One is still fast and powerful, and outside of that one Unreal Engine 3 game, has no problems with any app. So my point is that even though other devices have better hardware, rarely is the Nexus One outdated.
And of course there's the march of progress, and the nexus one of one year ago is now outmatched by almost all devices with better screens, dual cameras, coming dual core processors and much better GPUs, etc.