These are not mutually exclusive things, as superiority of the systems is a multi-dimensional metric. I quoted cap'n proto as an alternative to protobuf that I would definitely choose over any protobuf, because in my book it does at least a few things better. Namely, the bits related to immutability & zero-copying, and random access. But at the same time I do not like and do not agree with your field optionality stance, as I think it is based on a false premise that a universal optionality is the only viable path towards compatibility. I will cite the original article regarding the matter to clarify this point:
> protobuffers achieve their promised time-traveling compatibility guarantees by silently doing the wrong thing by default. Of course, the cautious programmer can (and should) write code that performs sanity checks on received protobuffers. But if at every use-site you need to write defensive checks ensuring your data is sane, maybe that just means your deserialization step was too permissive. All you’ve managed to do is decentralize sanity-checking logic from a well-defined boundary and push the responsibility of doing it throughout your entire codebase.
This approach doesn't free you as a developer from having to maintain multiple code-paths as you claim to be able to avoid in your older comments ("you end up needing to maintain parallel code paths, leading to ugly and hard-to-test code").
Code paths are still there, they are now intertwined with your business logic as conditional checks on a field presence. At every calling site that utilises the schema.
That's one of the reasons why I prefer flatbuffers over cap'n proto when I have a choice, and it is the reason why I think that you are not fully aware of the issues that stem from the choices of protobuf and that are clearly manifested in ecosystems that model network communications via advanced type systems.
In fact, this comment from your linked thread suggests a similar idea - advanced type systems can provide a strict schema negotiation in semi-automated way, at a fraction of the effort required to maintain schemas with all-optional fields - https://news.ycombinator.com/item?id=18201601