QuestDB Head of DevRel here ... Yes, it can be a replacement of Postgres and it will be cheaper and faster. That being said, PGwire is still in alpha and is not 100% covered yet, so while migrating is possible, 100% Postgres Wire Protocol compatibility is not there yet.

For traditional transactional RDBMS data, I don't think it's a very common choice. For Time Series data, QuestDB is by far the fastest choice for Postgres-compatible SQL Time Series databases.