The main problem with "just" REST IMO is that it's only specific at a general level - so things like how to deal with whatever related data should or shouldn't be included are kind of "left as an exercise for the implementer." If REST were basically the same but included some kind of universal specification for managing fields, resources, nested data, and relationships requested/returned, or even allowed a means of going single-endpoint for request reduction, the many variants and alternatives, whether GraphQL, JSON:API, or others, wouldn't exist to the same degree.