You don't. I just moved a WP multisite from Rackspace Cloud to AWS. Load times down 50%. Cost down 90%. Hosting is not, and has never been, an easy business.
90% seems way off. AWS is known for not being cheap (neither is Rackspace), but I highly doubt that Rackspace was ten times the price of Amazon.
There are MANY applications that simply are too complex to handle such an arch... so, - do so, when you can, but know when that's just stupid.... stacks are like snowflakes, we LOVE them to death, or we love them to DEATH...
Not if you need always-on instances.