The fact remains that many android users are unable to upgrade because the carrier has yet to roll out the latest version of android for their phones
The first chat is the adoption rate of android for the first 12 months (source). Nougat has a paltry 14%
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The iOS adoption rate is completely different tale. In just 5 days, 20% of the people upgraded. The chart below is only upto iOS 10, I didn't want to kill myself for a similar one that includes iOS 11, but from my research iOS adoption rate is similar.
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As you mention this can be a big issue with security and patching vulnabilities. It also can be frustrating when you spend nearly a thousand dollars for a new phone and you have to wait 6 months, or longer for the carrier to roll out Android. Step down from a flagship phone and you may not even see an upgrade. That happened to me, on a phone, they promised, and then didn't roll one out.
It seems Android OEMs are doing better with monthly security patches, at least Samsung is in the states. OS updates are still pathetic. The bottom line with most Android OEMs is if you want the newest Android OS, you need to buy the newest flagship.