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!
The key point is, running those things yourself, you are likely to lose all the nice failover mechanisms/monitoring/auto-snapshotting stuff that AWS offers. To live up to that, it will require you to not only have extensive understanding of the software you are running, but also a considerable amount of your time will be dedicated to Ops side, which can lead to some really big frustration from time to time. In that sense, I don't think you can re-IMPLEMENT something by assuming too much of comprises.