> With things like Flatbuffers, I sometimes feel we've regressed to these old formats

Sorta, but they're expressed in IDLs that are independent of a particular compiler. The downside is the drift between internal structs and the IDL structs, but the upside is you can use the same "memory dump" on interpreted runtimes or entirely different platforms (even those with different endians). Plus impls like protobuf help guard against breaking backward-compatibility by numbering fields and not "allowing" you to remove fields in ways that would change the structs' ABIs.