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.