they have contracts to support the US military for many decades into the future. Hard to be more sustainable than that.
Until I read the transcript, just now, I had confused SQLite's origins (a General Dynamics contract with the US Navy) with its through-2050 support contract, which, it turns out, is with Airbus.
Either way, that doesn't make it "closed source" like you said in the other comment.
Anyone on the planet can make changes to it and distribute those changes. The only thing those 3 can do that the rest of us cannot is get their changes into the copies distributed at sqlite.org.