JSON contains a description of the structure of the data that is readable by both machines and humans. JSON can certainly go wrong but it’s much simpler to see when it has because of this. GRPC is usually a binary black box that adds loads of developer time to upskill, debug, figure out error cases and introduces whole new classes of potential bugs.

If you are building something that needs binary performance that GRPC provides, go for it, but pretending there is no extra cost over doing the obvious thing is not true.