In this scenario it actually does cost Apple nothing.
This is largely irrelevant. If the developer of the third-party software wants to provide it for free, a one-time cost, or a subscription, none of that matters. None of that has any affect on Apple. It is the third-party developer's choice.
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And because I know someone will comment, "But Apple created all the APIs." Yeah, they created APIs. So do many software developers. Apple's APIs benefit Apple two-fold:
- Their own software uses them.
- Third-party developers can make software using them and provide a more unified experience to the end-user.
Apple does not need to charge for providing APIs. Providing APIs allows developers to more easily write software for Apple's platform. More available software makes the platform more attractive to end-users.
All of this comes down to a single thing: Apple wants money. They want money from every one at every stage for every thing.
- They want money for the original product.
- They want money for any and all upgrades to the product.
- They want money when the user needs to repair the product.
- They want money for any and all software that runs on the product.
- They want money for any purchases made on the product.
- They want money when the user is done with the product and ready to move on to a different product.
They really only deserve money for the first one, but they structure their products, services, and policies with the end goal to extract money in all areas, even if they had absolutely no involvement in the transaction.