It's unfortunate that they went silent for years after the Apple acquisition. That period was key for database adoption. I have the feeling everybody kind of settled for pgsql.
That's probably because of spending time on this echo chamber.
In reality everyone has likely been staying with the same databases they know and love but just moved to the cloud. It's why now AWS for example offers such a wide variety of databases e.g. MySQL, PostgreSQL, SQL Server, Oracle, MongoDB, Cassandra, Redis.
Of those the first two are not PG, they just share the wire protocol and try to be compatible at a SQL level. Aurora is not really distributed, it's replicated for availability and durability six ways at the block storage level. Citus is distributed as I understand it though.
> but it's also reasonable to come at it from the other direction - build a SQL engine on top of a solid distributed key-value store.
Sure, that's possible. I'm not talking about the wisdom of building your own relational database as the end goal, just that a distributed key value database and a SQL database don't have overlapping problem sets.