So when the moment comes and you want to move away from AWS and you have scaled well beyond the capacity of a single box, what then? Suddenly you have to figure out sharding, replication and HA setups from scratch. While that may be the right choice for you if you're an early stage startup and it lets you defer engineering costs until a point where you're hopefully better funded, it's also dangerous:
It's far easier to get things like sharding and caching and use of read-replicas and multi-master replication right if you think about it from the beginning (if you've ever tried to retrofit any of this onto an application not designed with it in mind you know what I'm am talking about). It'd be awesome not to have to worry about this, but if you don't, you tie yourself into a platform you'll find it harder and harder to move off.
As a product, this solution would be a killer - it sounds fantastic. As an AWS-only service, I for one will treat it as if it doesn't exist, because getting tied to AWS for the long term is not an option for most of the projects I do.