> The goal is not to have a single source of truth schema. That is a means to some other goal, and it's not even a good means.

It’s about data transmission. Being able to encode and decode in a type safe manner between different languages (and so, different platforms) is a goal that makes a lot of sense.

> If you do change your schema from time to time, you need testing between the two systems

Or you could just use a defined format that doesn’t require testing. I rarely use protobuf but I can see why people do. The guaranteed backwards compatibility is huge for people who can’t just publish a new web frontend at the drop of a hat.