I would also like to know why you don't consider it 'modern' and how you would define modern-ness in this context.

I answered above.

https://google.github.io/flatbuffers/ and https://capnproto.org/ are both successors to protobuf

I don't think they are successors, just different. Protobuf is a sparse wire format, whereas FlatBuffers and Cap'n Proto use a fixed-layout wire format. There are plusses and minuses to both. I wrote a little bit about this here: https://news.ycombinator.com/item?id=6329041#6330426

(Disclosure: I work at Google on the protobuf team)

gRPC uses Protobuf version 3. Both it and CapnProto are successors to Protobuf version 2. Flatbuffers is not a successor but is targeted at a different use case.

It's easy to argue you are just rattling off buzzwords if you don't provide examples.

I would also argue that not based on a schema is even more brittle as you end up implementing validation logic and client marshalling that the schema would allow you to just generate.

Name some. Seriously, I'm interested in protocol design, so I'll read them. But also, yeah, just declaring that things exist is kind of unhelpful.

> Such things exist, I'm not just rattling off buzzwords.

you are though ...