As far android is concerned, sideloading is allowed but at the cost of lucrative play services SDK. And play store implements rules haphazardly.
App stores are platforms where softwares are deployed. While creating a sustainable business model, it's necessary to study the deployment platform and implement the suitable 1 business model.
One of the best case studies for multi platform deployment is alto's adventure. It's paid on iOS and free on Android. They know the platform and target audience and implemented pricing models with the platforms on mind.
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