For typical users not buying Apple means having to compromise on privacy with Google, which isn't a great option either. Both are trying their best to create vendor lock-in and make it hard for users to leave.
From developer perspective not having access to iOS users is a major problem. Apple inserts themselves between users and developers, even where neither users nor developers want it. Users and devs have no bargaining power there, because Play Store does the same, and boycotting both stores has a bunch of downsides for users and devs.
*) I expect people on HN to say that some AOSP fork with f-droid is perfectly fine, but that's not mainstream enough to make Apple and Google worried, especially that Google has created its certification program and proprietary PlayStore Services to make degoogling phones difficult.