SQLite has been taken seriously by serious people for decades. We just don't hear about it in the cool, hipster startup circles because it isn't sexy. But it's steadily grown to be the most deployed embedded database in the world, in millions of devices, and have specialized strands that went through military and medical grade formal validations to end up in airplanes, fighter pilots, naval ships, medical devices.

You have to look where SQLite isn't rather than where it is these days.