I think you might be confused somewhat...PostgreSQL doesn't have "
a vendor". The history of Postgres starts at UC Berkeley and now has the PostgreSQL Global Development Group which is a mixture of contributors both community and corporate sponsored:
https://www.postgresql.org/community/contributors/
Of that group, both 2nd Quadrant and CitusDB are represented...so in a way you could say their support is "by the vendor". Not to mention EnterpriseDB which also has support options.
> we can't use CitusDB when it is not supported by other vendors for use with their products.
CitusDB is no longer a fork, it's an extension...this means so long as your other vendors products support Postgres, they support CitusDB. More over, CitusDB itself will sell you an Enterprise package.
> The point still remains that after all these year PostgreSQL's scalability story is still a mess.
If by mess you mean specifically there is no knob and dial arrangement in the core of Postgres I would agree.
But within the core of PostgreSQL there are primitives which make scaling approachable (I myself am working on a data ingestion process that utilizes table partitioning and hand rolled sharding for PostgreSQL).
And there are now a plethora of extensions and tools provided by core contributors such as 2nd Quadrant and CitusDB to offer somewhat out of the box solutions and they even come with support.
PostgreSQL is not the right tool for every job and the replication/clustering area has been a a sore spot for Postgres in the past. But it certainly isn't bereft of options now...and the inclusion of logical replication in this beta is only the first step in bringing these options closer/into the core.