Security, privacy, reliability and “it just works” was always a major selling point of the Macbook too.
The dichotomy between secure and locked and open but unsecure is a complete strawman. It is possible to be both open and secure. People would still be able to only install applications from the App Store even if the iPhone was less locked.
Apps on Mac OS X can absolutely screw up your machine. Adware in particular is a big problem for novice OS X users (speaking first hand here, having had to support these users). That’s an exceptionally rare occurrence on iOS.
I’m comfortable with the security model on macOS, but would be very concerned if that same model were applied to iPhoneOS, where users (myself included) carry significantly more private information.
On my phone, I want applications to have access to as little private information as possible, even if it’s detrimental to functionality. Call me a security nihilist, but security and privacy is by far my #1 overriding concern on mobile platforms, given the wealth of personally identifying information these devices gather.
More succinctly: the threat model on mobile devices is not the same as the threat model on personal computers. It’s inappropriate to apply the same security to both platforms.
And regardless, Apple has never hidden the nature of iOS. These consumers have clearly decided that the tradeoffs of the walled garden were acceptable, despite there being plenty of more open alternatives on the market in the early days of the iPhone.