Relatively few startups need scaling all across the stack. For instance, some portion of folks who sell downloadable software have bandwidth issues but largely uncomplicated websites.

For those folks, you can recommend S3 and tell them "This will cost you essentially nothing, it will fix your problem, and integration into your existing site will take ten minutes if you type slowly."

What is the migration path from a successful business to the App Engine?

Step 1: Learn to write Python.

Step 2: Forget everything you've ever learned about databases.

Step 3: Get a cup of coffee. You're not anywhere close to done.