If we've lost access to the schema, then we've lost access to our entire source repository, so we probably don't need to read our data anymore?

I guess I could see that being an interesting property in software that wants to deal with incompatible forks, or software that for whatever reason wants other people to read their data but is unwilling to ship a schema with it, but just for persistence on backend servers it doesn't seem like a very interesting property or worth the extra space requirements.