Heroku isn't a competitor to AWS, AFAIK. They're a partner.
When someone like Heroku offers a competing product, the fact that the product is also being run on EC2 is only a part of the overall story. Over time, AWS will probably be less and less happy with earning just the bare infrastructure dollars for such use.
Just build a HA RDS Cluster yourself with Click and Scale, than you can sell it to me with ~25% higher prices than the EC2 instances. Btw. AWS RDS is cheaper than most other competitors like ElephantSQL.