Apple has been kind of forced into the open source model by tools like Corellium[1] and some source code leaks. This means that the black box advantage that iOS has had is no longer as significant. Apple's move with the Security Research Device Program[2] proves this point. They wouldn't be offering it if they didn't think it was a serious problem for them.

[1] https://www.corellium.com/ [2] https://security.apple.com/research-device/