>As an iOS developer I can't use new OS features until all my users have upgraded. As long as what I need is in the support library I can adopt the latest and greatest for Android right away.

I don't know about the rate and speed of support library from the Top 5 Android manufacturers, which is Samsung, Huawei, BBKs, Xiaomi. I know they do get some regular update, but on all devices? And do those user actually update it?

iOS Users Update are on a much faster cycle. iOS 12 is on 70% of compatible iOS devices within 3 months of release. iOS 11+ on 95% of all compatible iOS devices. That is pretty damn crazy if you ask me Having OS that is only 2 years old on nearly all devices.