Would be interested to hear more about your opinion on why using a database is a mistake.
Using a datastore with a black box query planner that explicitly doesn't allow you to force particular indices (using hints or similar) is a more subtle mistake but will inevitably bite you eventually. Likewise a datastore that uses black-box MVCC and doesn't let you separate e.g. writing data from updating indices.
We are also working on shifting some workloads off of Postgres on to more appropriate systems as we scale, like logging. But we intentionally chose to minimize dependencies by pushing Postgres further to move faster, with migration plans ready as we continue to reach new levels of scale (e.g. using a dedicated log storage solution like elastic search or clickhouse).