Not only that, but with today's homogeneity in hardware, the phone market takes some time to shift. I held onto my Android phone for my two year contract and replaced it with a Lumia 920. My wife's Nexus contract is expiring in a few months and she is considering a Lumia as well. That transaction will occur about a year after my last upgrade.
These things don't happen instantly now that so many devices are alike.
By comparison, when we adopted the first iPhone, it was a dramatic departure from previous cell phones and we had to have it immediately. Since then, we've just waited for our contract terms to expire.
Though I would interrupt my contract immediately if someone would deliver on personal application omnipresence, making a cell phone that is just a small touch-enabled view on my network and my applications.