Cassandra in particular doesn't compete with RDS as much as DynamoDB and, respectfully, setting up failover and monitoring and autosnapshotting isn't actually that difficult. I'm a big fan of outsourcing that stuff--the new system I'm building uses AWS Lambda, for example--but when using a managed solution requires compromising on features or tying yourself into knots to make things work, hosting your own is doable, especially in the age of devops. Automation is your friend!