Enforcing permissions and auditing third party code are two different things. The point of app permissions is that when you know an app should never do something, you constrain it from ever doing that. Then if it has a security vulnerability, it's limited in the damage from the resulting compromise.

The people configuring the sandbox should be packagers that you trust. The upstream developers might provide some recommendations to the packagers, but if it's obvious that an application shouldn't need a permission, it shouldn't have it.

But permissions can't save you from an actually malicious app. Constrain it from accessing the camera and it will still be using your device to host pornography. Constrain it from accessing the filesystem and it will still run up your electric bill mining bitcoin. You either need to trust the developer or you need to get the app through someone you trust to have audited it for you.