I get what you're saying, but these JSON features are actually indicative of a deficiency in the relational model: The relational model has no performance mode

I get what you're saying, but these JSON features are actually indicative of a deficiency in the relational model: The relational model has no performance model. The existence of ever more complicated query optimizers proves this. This is fundamental engineering issue, and NoSQL and JSON in MySQL are engineering hacks that address this issue for specific problems. It's also a big usability issue. Even if you can tune your queries, indices, and schemas to remove a given performance bottleneck; it could be beyond the skills of users. NoSQL and JSON are indeed simpler. So it would be nice to come up with a better model instead of one-off hacks, but it's hard. I think it would be cool if you could exhaustively enumerate the queries an application makes, and then the database would somehow generate the schema and indices for you, and also give the time complexity bounds for the queries. But there is kind of a chicken and egg problem there, because the queries depend on the schema.