iOS 9 installed on over 50% of users' devices
apple.com
apple.com
Apple sold 1 billion iOS devices as of Jan 2015. They don't all support iOS 9, but looking at the cumulative sales graphs, let's just say there 700 million iOS 9 compatible devices.
Getting 350 million people to download and install software on 350 million devices in 5 days is pretty freaking amazing.
Most software autopatches these days. Is this any different than the latest version of Chrome installing itself and working almost flawlessly on a huge number of different hardware platforms (rather than < 100 distinct device types)?
That's not to say that upgrading to iOS 9 is the same as forced labor, but when you control the market as tightly as Apple does, and then fan the flames of the "the new new thing" as something you have to have ("nothing has changed except everything"?), yeah, it's not gonna be a really hard sell.
I think the most impressive part of it is that a lot of people are reticent to upgrade due to fear of bugs, and it's a testament to Apple that the vast majority of folks trust them to roll out quality releases. Of course, it could be their audience doesn't give a shit about quality and just wants the new new thing. Not really a single variable study here.
I think 9 doesn't bring a lot of new features, more bug fixes and refinements. If 8 is that bad 9 might be worth a try after a couple revisions.