> People who advocate paying the higher cost ahead of time to perfectly type the entire data structure AND propose a process to do perform version updates to sync client/server are going to lose most of the time.

that's true. But people also rather argue about security vulnerabilities than getting it right from the get-go. Why spend an extra 15 mins effort during design when you can spend 3 months revisiting the ensuing problem later.

Alternatively: why spend an extra 15 mins on protobuf every other day, when you can put off the 3-month JSON-revisiting project forever?