Does nobody in you family own MacOS or Windows? Because there you can install malicious apps so your point is selective. Also there were many malicious apps approved by apple - this days they do a lot of checks automatically so nothing prevents to do it (kind of like virus or malware scanner that would check something before installation).

If you worry about your family why apple cannot provide some similar but simpler tooling like parental control where (non-tech savvy) family will have to ask you for confirmation? In his situation you would be some administrator or guardian but will much less permissions.

Why bother? It's win win for user and developer because they can offer you something cheaper. They can advertise you that if you buy subscription from 1st hand it will be cheaper.

It would also allow different business model - instead of monthly / yearly subscription you can charge for app one time and then maybe for big upgrade every few years.

But it's not only that about the cost. You would be allowed to have apps that are not possible because would be rejected or cannot use private API that apple apps have. It would allow to have browser that support fullscreen mode, push notifications like android, better PWA support, WebGPU support. Faster cross platform apps (react-native) that allow access to JIT. 3rd party keyboard that allow much better voice transcription even locally (all 3rd part keyboard have it kind of broken right now because of API limitation) and the list goes on.