Nice catch: Building something for one purpose then [gradually] end up using it for something else.

For software you probably want a feedback box baked in.

If there should be something like star ratings it should probably be baked into the app and allow the user to change it at any time.

You probably get better numbers by measuring how long the application remained installed and how many minutes it was used in total.

As a user I would be interested in the volume of instant uninstalls. For games you want many hours of usage, for simple tools a very low number could be attractive.

OT: You had me imagine a music store with in-song purchases. Give the buyer all instrument tracks so that they can adjust the volume of each, buy extra instruments and singers, crowd source new instruments. Seems fun.