I'm on Republic Wireless, so there's only three phone models. They did a 4.4 to 5.1 upgrade fairly recently on every model. There were a lot of people begging for the upgrade, but there was also a very vocal group of people complaining about the upgrade, either the process of doing the upgrade, or the fact that so much in the UI changed that they were comfortable with. A lot of people were asking to be able to downgrade because they didn't want to re-learn how to use their phones.

Statistically, I don't know what percentage of people want upgrades vs don't, but anecdotally, I feel that most non-technical users don't want upgrades because they've learned how to use the phone and they're comfortable with it.