I do feel Amazon should have open-sourced this kind of technology. At work, we have hit the problems with MongoDB which many people face:
If such maintenance pain points are reduced by a managed service, it does allow developers to be more productive.
However, the article mentions that DocumentDB is based off of Mongo from 2 years ago! That is a cause for concern.
Really feel that technology with this kind of promised impact should have gone the Kubernetes way and enabled everyone to use and contribute. Maybe this pushes Google to follow suit, albeit the OSS way.