This is Amazon's wet dream. Your app isn't an app at all, it's just a collection of configs on the AWS Console. When and if the time comes to migrate off of AWS, you realize you don't actually have an app to migrate.
Also, Lambda-like options are available with most PaaS providers now.
It is not much different and might be easier than migrating web app from a custom PaaS. The only issue is, and has always been, the migration of data. And I don't see that getting solved until some startup writes a bunch of layers on top of a bunch of providers. It's very tricky, for good reasons.
Which other ones are there? I used to use PiCloud until they were bought out by dropbox and it atrophied. Shame, it was exactly what I wanted in a service.
We launched a month before Amazon Lambda, and have better features like full support for streaming HTTP.
Still in the process of establishing our service tiers, but our basic hosting plan starts at $5.00 per month.
IBM Bluemix - IBM Workload Scheduler, RabbitMQ (this is weak, but similar functionality can be achieved)
Microsoft Azure - Apache Storm (generally available now)