What I find especially odd is that they have built their API-compatible replacement for a Google property on another Google property. Although App Engine is unlikely to see a shutdown in the way Reader has/will be, they're still at the whims of a company whose primary business isn't a development/app platform.
You can't put out blanket statements like that. Google is a huge company with many different divisions. You may as well just impose a ban on Google on not even use Google search. It's like saying "Don't use AWS because Amazon is a commerce site".
Also App Engine is a paid for product. When you decide to charge for a product things become 'real' because you can't just shut down a product people good money for and rely on for business. Of course in theory anything could be shut down but the likelihood greatly decreases once you start charging users.