> SQL is the only programming language

SQL is not a programming language. You do not write programs in SQL. It's a declarative language (or set-of-sublanguages).

> a working developer can learn once and > use for 30 years without rewriting their mental model.

There is any number of long-living languages which satisfy this.

Plus, SQL it's not even really a single language, because the spec changes, and is huge, and few people know it fully; and the dialects have non-trivial differences; and if you switch DBMSes, you often switch SQL dialect. In that sense, it is very much like other programming languages which evolve, like C++ or Fortran or even C.