"Walled garden" does not mean "safe system". And it is not a prerequisite for a safe system, or vice versa.

You are saying you are happy in a "safe secure system".

In contrast, a "walled garden" is a prohibition on alternatives, not a source of safety. The prohibition of alternatives does not make the App Store safer.

If anything, it protects Apple from competing with safter alternatives! Like an app store only for children. Or an app store of formally verified apps.

Please correct me if I am somehow missing something...