I think maybe we're using "API" differently, here? GraphQL lets you query and mutate an object graph, but things exist that can't be (canonically, compactly) represented as a tree of objects. And usually at least a few of those things are necessary for any given API.

For example, how do you implement your AAA login logic using GraphQL? You don't, right? Logging in isn't really anything to do with querying or mutating an object tree. (You can deform your logic into making it so, but you'll be breaking things like the ability to do OpenID Connect.)

How do you implement an endpoint to stream chat message events in GraphQL? You don't, right? You want a stream of trees? A tree of stream-poll-results? There's an impedance mismatch here.

You want to use GraphQL to upload attachments? Use GraphQL for webhooks? Use GraphQL for Operational Transformations in a Google Docs-alike? No, no, no.

GraphQL is useful for implementing the message-format of particular endpoints of an API. It doesn't work for all endpoints. Unless your API is entirely just about a particular object graph, you still need to put your GraphQL inside a larger abstraction that distinguishes the GraphQL endpoints from the other endpoints.