iOS has hit double digits. My biggest question going into WWDC was whether Apple would treat the number 10 (and the simultaneous removal of that number from macOS) as a milestone or just the next increment in a yearly series. The answer is that iOS 10 is a big release, but not a big departure in the way that iOS 7 was. iOS 10 is not Picomac-sized; Apple called it "the biggest ever" without hyperbole; the MacStories gang have already put together a 5,000+ word article just covering Monday's preview.
The keynote broke down ten hallmark features, leading with the nebulous feature of "user experience". I think Apple could have left it there if they weren't trying to play on the number ten. iOS 10 looks like a redefinition of what it means to be an iPhone user. Note that just now, and in the title, I didn't say "iOS" or "iOS devices". Every new feature and demo was presented in the context of the phone. Even more specifically, iOS appears to be designed for the iPhone 6s family and beyond, with tons of functionality accessed by 3D Touch. There appears to be no alternative for iPhone 6 or SE users. That would be a totally ordinary, forward-looking Apple move: the latest OS runs best on the latest flagship hardware, so upgrade early and often.
Except what of the iPads Pro? The most powerful iOS device available is only months old. iOS 10 will perform beautifully on that hardware, but possibly lacking major features like inline widgets and interaction with notifications. I had hoped that iOS 10 would give me clarity on whether to purchase a 9.7" or 12.9" iPad Pro this summer. Instead, it's made me question whether it's the time to buy at all, even though my 3rd generation iPad has now officially fallen off the back of the conveyor belt, as it doesn't support iOS 10. (Mine's actually still running iOS 7, just to keep it functional.)
I may have to beta test my purchase decisions with the iOS public beta program this summer. I can take the plunge for the first time and put the beta OS on my carry phone: an iPhone 6, which lacks 3D Touch. If I feel like I'm not getting the full package, it will confirm my decision to upgrade the phone this fall (a little less than two years in), but it may prolong my iPad limbo.
iOS 10 looks huge for the direction of the platform writ large: iPhones, iPads, (sorry, iPod Touches), and the four-headed system software monster. As long as the hardware keeps up, it makes the argument that saturation does not entail stagnation. After all, it's only the first double-digit iOS.