What you want is basically a recipe for the mobile web turning into an exclusively corporate wasteland. Lack of installation freedom doesn’t provide security from anything when the A/G app stores are already full of malware. Real security lies in the phone owner being able to exercise choice if he wishes - to use a FOSS app, or to pay conventionally via the web instead of having to put up with opaque portal.

Phones are generic computing devices. We must able to operate and maintain them however I wish.