I'm wondering what the point of all of this is.

If you can afford the large fixed cost of vertical integration, it's always cheaper to do things yourself, so the sweet spot for using providers like AWS is scaling down, not up. A managed DB lets you hire a fraction of a sysadmin or devops person from AWS.

The moment you end up paying enough to AWS to hire a sysadmin, you basically are getting an antagonistic sysadmin from AWS, whose primary goal is to make as much money off you as possible. The incentives are not aligned.