So, as you scale up and out in size, you stop being able to depend on higher level abstractions from elsewhere. You can't build a Tier-1 service like S3 or EBS

So, as you scale up and out in size, you stop being able to depend on higher level abstractions from elsewhere. You can't build a Tier-1 service like S3 or EBS on top of RDS. If you need a database solution to help you with building and running tools of that scale, it has to be something else that is a lot less complex and a lot more robust. It's okay for RDS to be dependent on a Tier-1 service like S3 or EBS, but not the reverse. You don't want to get into priority inversion problems here. And when your service has to be in every AZ in every region, and they all have to be kept separately running, then things get even more complex. Then you have to ask yourself what is the bootstrap process for building a new region.