"...hard to imagine scenarios where [file-level compatibility] is useful" what am I missing? Surely dropping a more performant dbm into an existing project would be the application? No?

Could you imagine a project that is simultaneously:

1. having such a huge local SQLite file that migration to another format is unfeasible;

2. bottlenecking on a single specific metric that happens to be more performant in Turso;

3. ready to introduce another programming language and its build toolchain into the project;