> We have general purpose databases. They're called filesystems. You can literally store anything and everything you like, and hardware is ever-shrinking in cost and performance.

And they're even more limited than relational databases when it comes to the ability to scale out, or have good consistency semantics without deadlocks. Just implementing a basic mutex on a (general) filesystem is nigh-on impossible. And while AFS is impressive, there still aren't really any true master-master filesystems, much less filesystems that can autoscale.