If you do json rpc over HTTP/2 the performance will actually not be that different. The serialization layer is different and for sure protobuf will be fast to (de)serialize than JSON, but for most applications serialization is not the bottleneck (you can e.g. bury a lot more performance in the HTTP implementation).
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Significantly less overhead and the benefits of Protocol Buffers on top of that.
If performance isn't a big deal, JSON-RPC over HTTP or something like it is fine. If it's critical, something like gRPC makes more sense.
Apart from the performance gain, the generated stubs (and the generated protobuf classes) are a nice improvement in developer experience.
If you do json rpc over HTTP/2 the performance will actually not be that different. The serialization layer is different and for sure protobuf will be fast to (de)serialize than JSON, but for most applications serialization is not the bottleneck (you can e.g. bury a lot more performance in the HTTP implementation).