A while ago I was thinking the same thing Matt, and I explored an implementation of a GraphQL-inspired endpoint built on top of the WP REST API infrastructure introduced in 4.4 in this Github repo: https://github.com/BraadMartin/simple-graphql-api

It doesn't require Node or any dependencies aside from WordPress 4.4+, and while it doesn't even come close to implementing the full GraphQL spec, it gave me a sense of how some of the ideas in GraphQL can be used with the existing infrastructure we already have in WordPress core. Definitely fun and potentially useful for specific use cases where a traditional REST API gives you too much data or you really want full control over the API response from the client side.