JSON is the best, simplest, most robust, most readable, most flexible data interchange format ever invented and fastest to integrate with external systems. No way I would give it up for Protocol Buffers - That's like going back to the old SOAP protocols.

Have you tried integrating with an external service via a SOAP API? It would be quicker to rewrite the entire service from scratch than to integrate with it.

Also, I'm not a big fan of gRPC; their decision to use HTTP2 instead of WebSockets was a huge mistake. HTTP was designed to transfer hypertext documents; RPCs are meant to transfer raw data; not hypertext documents. In a way, gRPC makes the same mistake as TypeScript; instead of fixing the problem at the root, they take an already complex solution which works well as a base and then they add more complexity on top in an effort to make it behave in a simpler way than the underlying technology.

You can't make something simpler by adding more complexity on top. You just can't.