FlatBuffers can be used with gRPC, so they shouldn't need a separate RPC ecosystem like Cap'n Proto: https://grpc.io/blog/flatbuffers
Disclaimers:
1) I'm the author of Cap'n Proto; I'm probably biased.
2) A lot could have changed since 2014. (Though, obviously, serialization formats need to be backwards-compatible which limits the amount they can change...)