(Author here) Couldn't agree more on not turning technology choices into religious devotions.
It is of course completely possible for you to build a query language from grounds up. django-restql seems to be doing this. Sparse fieldsets and Netflix Falcor other alternatives I've heard of. GraphQL however has good tooling and if you do see the need for a query language on top of your APIs using it is a good choice IMO.
GraphQL does not necessarily mean you have to throw away your existing backend and rebuild from scratch either. It's perfectly fine to use GraphQL as an additional API endpoint alongside existing APIs. A model we are seeing evolve with Hasura is to use Hasura for all the reads and delegate writes via the Actions framework either to a serverless function or another HTTP service.