When Apple decides to take away an app I paid for, I have no security to block them. When Apple decides to quit posting security updates, I'm out of luck. When Google reaches in and takes my location data without permission, I can't stop them.
With open software, there's no guarantee I'm more secure. But when some security issue does come to my attention, I'm more likely to have some say in how I respond. Somebody claims to have a more secure or privacy-respecting driver? Strongly recommended by experts I trust? I'll look into it. With Apple, I can't. There is no such alternative.
The ssl lib has known vulnerabilities? Wait for Apple? Or install some shim until there's a fix? Oh wait, on Apple devices, you don't have an option.
Apple is not only closed. It's walled. As the owner I am kept on the wrong side of the wall. And there are parties on the other side of the wall that I don't want to be on that side.
This is a security model that is hard to lose to. If Purism let's me control the wall, I'd say that's a welcome change.