There are tens of thousands of engineers maintaining and supporting postgres and clickhouse in all the organizations where they are used, the vast majority of which are not on Twitter's scale.
Not tens of thousands at any one organization supporting postgres and clickhouse.
No single organization needs even hundreds of people to support these apps. You just need a good architecture and a handful of dba's, developers, and sysadmins... maybe.. depending on your scale. At many smaller orgs you can probably get away with one.