This works until you have to integrate with third party solution to exchange data, then you end up needing to "externalize" the data, and why not from there sta

This works until you have to integrate with third party solution to exchange data, then you end up needing to "externalize" the data, and why not from there start with protobuf/flatbuffers/capnproto/etc. For each of these one can rewrite a more optimal (perf sensitive) writer, but missing some functionality (e.g. append/write only protobuf with zero overhead). Granted, for their task this was over complication, but if they were doing ads (or something similar) it always needs ways to structure your data, and then you end up needing something hierarchical, typed, extensible with backward and forward compatibility (e.g. in protobuf for example, never reuse already used index field, etc.)