The database market will all its competition is definitely challenging. I have no doubt AWS will increase their database market share over time. The good thing about this competition is that it is forcing all vendors to be innovative and to find (more) USPs.

AWS DocumentDB seems to be pretty much tied to the MongoDB API right now... So At the moment this will somewhat limit its functionality. However, they will not stand still and probably also extend into the multi-model space at some point. Apart from that, not everyone will be willing to pay for DocumentDB or have their data located in Amazon datacenters.