I think get this now - with SOAP/RPC/REST we can establish the interaction contract by explicitly defining inputs and outputs. With that contract, we can draw clearly defined system boundaries, give better assurances to the developer that they are doing the right thing, and improve testing.

You're right - in the scale of app-level problems, GraphQL is just a low-level protocol. It requires language-specific libs to address these higher-level problems (provide interfaces, structs, query typing, etc). As a very basic example, programatically building a GraphQL query is unnecessary complex outside of JS because many languages don't yet have a QueryBuilder for the GraphQL syntax. There are some community efforts, but as of yet no heavy corporate players to drive GraphQL-specific dev (e.g. like Pivotal/Sensio/Sencha did for their respective fields).