In many ways, SQL is the protocol by which we combine programs together.
It's true that it happens, but it's the antipattern of our age. When you have seven applications written against a single database (plus a load of complex SQL that operators use for support), it becomes impractical to refactor the schema. Now your business is evolving around rotting data structures. Companies like Oracle make their money by convincing people that they should adopt this misdesign, and then licensing e…
Relational theory is intended to make it easier to achieve data independence, including various kinds of schema changes.
You did not offer an alternative, but I am very skeptical that whatever alternative you have in mind somehow improves matters.
"you can't stream data effectively"
If the currently available SQL products don't allow you to stream data effectively, and that's what you need to do, then use something else.
My point was what people are actually doing for IPC. And for the most part, it's using SQL databases. Even communicating between processes in a single application (e.g. related web requests served by different processes), SQL databases dominate.