> Protobuffers are shit coz they don't support zero copy and you have to deserialize the whole thing even if you are interested in one field or an outer envelope, causing memory churn in your JVMs. Cap'n'proto and flat buffers attack this real problem. The expressivity of the type system is a minor issue, hence no credible competition.

I'm not terribly familiar with protobuffers, but I'm a little surprised by this. Some ASN.1 encodings (BER, CER, DER), by contrast, use nested tag-length-value triads. This allows you to skip parts of the message that aren't interesting. (This is, by the way, not that uncommon.)

> Required should be enforced at application later not the binary packing layer. It is a property of the version of the code processing the blob, not the blob representation itself.

This might depend a bit. Suppose you wanted to make sure that messages could be read (if not exactly decoded) without a schema. The structure of the message could, in principle, include this information in the form of bit-field preambles that indicate the number of fields, extensibility, and so forth.

I don't suppose that's strictly necessary for most applications: embedding message structure in message content seems like a bit of an anti-pattern, but I bet you could come up with a use case that makes sense in some context.