This is a good point. Especially if you only cared about how fast you can make your app. But if you want to also consider how cheap you can run your app, you need to consider how many app servers will it take to saturate the DB? 1 or 10? At certain scales for certain tasks, the hosting costs matter more than the development costs.
>But if you want to also consider how cheap you can run your app, you need to consider how many app servers will it take to saturate the DB?
Moore's law has made this sorta moot. Unless you're on Heroku, for a successful small-to-medium app, the denominator in your hosting costs is doing to be the salary of the engineer or sysadmin who tends to it.
(If you're on Heroku, then you start worrying about dynos because, with monitoring, you're paying $60 per "worker".)
This is to say, the cost in salary to properly shard a database probably outweighs a year or two of hosting for the extra two or three boxes you're spinning up; almost no one experiences explosive growth where you need to spin up dozens of new boxes overnight.
Moore's law hasn't made it moot. Running in the cloud is pretty slow and extremely expensive. Look at StackExchange for example - they used to handle a LOT of traffic on a handful of servers. Even these benchmarks say (or said) that the EC2 instance used is waaay slower than an i7 2600K.
