You are assuming that 95% of the jobs will remain the same throughout their entire lifetimes. Which is not true and I'm sure many would prefer for a database not to become a liability in case they happen to attract a lot of users, which requires more availability than a classical DBMSes can provide.

The problem is more about constrains required to scale and achieve high-availabily and low-latency, rather than a database choice. But classical DBMSes lack those constrains and you may and probably will end up with a design that just cannot be scaled without removing and redesigning a lot of things that users rely on, and also with engineers that never learned how to do that, so you may end up in an urgent need of a distributed systems specialist too. Scary. You can avoid that by using systems designed for high-availability from the very beginning, like Riak. Same goes for blobs and filesystems, using POSIX API instead of an object storage is going to haunt you later on, because in your designs you may end up relying on things that ultimately cannot scale well and guaranty latency, as gitlab learned the hard way.