The reason reason people overpay for large databases on RDS: RDS spend makes little difference to a lot of companies when the alternative is hosting it in-house. AWS provides multi-az failover and ability to add read-replicas with a click of a button. The support is also pretty good if you pay for the premium tier: I've had a RDS support member join major database migrations (might not happen for smaller companies, but we are talking about large spenders here).