Very good points. The proprietary test suite is clearly the (open) secret to SQLite's success. It seems to me that it isn't even entirely accurate to describe SQLite as written in C when the vast majority of its code is probably written in TCL that none of us have seen. It's more like C is just how they represent the virtual machine which is described and specified by its tests. The virtual machine exists outside of any particular programming language but C is the most convenient implementation language to meet their cross platform distribution goals.

If someone did want to carve into SQLite's embedded db monopoly, it would take years to develop a comparable test suite. This seems possible, particularly if they develop a more expressive language for expressing the solutions to the types of problems that we use SQLite for. Who would fund this work though when SQLite works as well as it does?