Honestly when you list the merits of Postgres like that you sound like a used car salesman. "Of course you think you want that but really you want all of this." We all get that Postgres is great. Saying "no we won't implement a feature you, our customer, wants" like this just rubs me the wrong way.
Is there some technical reason that you can't offer both?
What would you like Heroku to do with a MySQL on EC2 offering that Amazon is not currently doing with their MySQL on EC2 offering (Amazon RDS)? They are both local, use the same underlying infrastructure, and have Heroku add-ons, no?
Presumably, their decision to only support Postgres is partly competition (many others provide MySQL on EC2), and partly technical. How would you justify the technical merits of the decision, without sounding "like a used car salesman"?
Sorry, I thought you wanted to know why we chose Postgres over MySQL.
Those are the specific features I think that make Postgres a better choice for both use as my day-to-day database and why we chose Postgres over MySQL for building a database service.
Certainly you could build a similar service on top of, say, RDS, but without the features listed above, I think it would take a lot more work and not come out as well.
Is there some technical reason that you can't offer both?