(1) You save a ton of money on bandwidth when you move data from AWS to AWS
(2) Your stack, in most cases, needs to be near each other to minimize latency. Databases get wrecked by this.
This is why, cloud database providers have to often transparently show you which cloud you're launching on [1] which effectively means AWS is going to get a good share of it anyway. My uninformed guess is that EC2&S3 are by far their biggest money maker which is going to be what unbundlers target.
I'm all for the unbundling and will probably take part in some of it, but I don't think it will be that easy.