For home and SOHO users, having a SQLite backend as an option would be advantageous: far easier setup, easier backups, way less administration and maintenance. I understand that the main concern is with concurrency, but SQLite only has that limitation for writes (one write at a time per database file). A solution that is mentioned in the SQLite’s website is sharding, for example by breaking up the database into separate databases, one per user. I would even suggest having one for the core system, and one per app.
This way, a SQLite backend would still be possible and usable/stable beyond testing purposes. Again, this would be a benefit for small installations only, like in Raspberry Pi, SOHO servers, etc.