Does anyone know the back story to Uber - why didn't it try to improve Postgres rather than move on to feed on another host?
Given the fact that the change which occurred is an "apples to oranges comparison" change (PostgreSQL and an SQL normalized DB table structure to MySQL and a noSQL style single table key-value store) then there is some credence one can put towards the rumor that some (or most) of the change may have come about from a PHB [1] saying "do this, this way".
[1] PHB (Pointy Haired Boss, Dilbert cartoon reference)
(note I said build, not use, different skillsets between driving a car and re-configuring the engine to run on seed oil)
Honestly, why would they? If there is a product that does what they need, why spend resources improving another.
Just because it's open source and they could spend money improving it, doesn't mean a company will spend that money improving it.
I would hardly say MySQL does what they need without any improvements. They built an entire second platform on top of it.
also their replication strategy looked like a joke and not enough automation. hopefully they never use galera, else their bad engeering practices could actually suffer in huge data losses.
I've once run galera-cluster and it pretty easy came to data losses especially after short network splits which occured randomly at the network.