You can have N frontends for one backend. If you need a new iOS app you will probably hire a team of iOS developers, not have all of your product teams learn Swift.

If your API looks like this...

  /ios-app/v1/landing-page
  /android-app/v1/landing-page
  /android-app/v2/landing-page
  /windows-mobile/v1/landing-page (legacy)
  /web/v5/...

where each platform has its own subtly different UI structure, the ability for frontend teams to get basically an arbitrary JSON structure of their choosing starts looking worth the extra work on the backend.

You might not encounter this problem at any point that's fine - there are other ways to avoid it, like having a cross-platform codebase. I have "hand-rolled" similar solutions to the GraphQL field selection before, and I would use the GraphQL protocol if I were to do it again today.