> Being able to look at JSON and understand it as a human on the wire is huge advantage

I don’t think this is a given at all. Depends on the context. I think it’s often overvalued. A lot of times the performance matters more. If human readability was the only thing that mattered, I would still not count JSON as the winner. You will have to pipe it to jq, realistically. You’d do the same for any other serialization format too. Inside Google where proto is prevalent, that is just as easy if not more convenient.

The point is how hard or easy it is for an app’s end user to decipher its file database is not a design goal for the serialization library chosen by Apple Photos developers here. The constraints and requirements are all on different axis.