I don't understand users like this. Your device is meant to updated. Not randomly sitting on some OS you the user have deemed the best. That isn't for you to decide.
Thankfully Apple has done a masterful job of implementing automatic updates, and update reminder nuisance, for users that simply ignore important things. 99% users will get the prompt, enter their passcode like a dutiful user, put their device on the charger, and wake up the next morning with as new OS. As it should be.
I can't decide what to do with a device that I purchased? Are you kidding?
Not updating to 10 because: 1) don't like the redesign of notifications, control center, music app and what not, 2) avoid the slowing down of my phone which doesn't have an option of downgrading to an earlier firmware, and 3) have no use for 90% of the new features, including all the gimmicky features of the new iMessage.
I can live with the potential 'security vulnerabilities'. After all, haven't had any issue of such sorts since I first started using the iPhone.
No, thanks Apple, I'll stay with iOS 9.