We migrated many of our critical protocols from JSON to protobuf, and the experience has been very positive. As your system gets bigger, and your teams get bigger, the value of a typed schema has a multiplicative effect on both reliability and productivity. In addition, a lot of basic features around managing transport, like load balancing, health checking, deadlines, etc. come out of the box.

IMO, performance is the least of the reasons to use protobufs. And if you're working on simple systems in small teams (or solo), then JSON is a better fit (because it's simpler to understand, debug, and iterate with.) gRPC is definitely not a one-size-fits-all.