They're probably going to end of life it like everything else they get bored with. I can't imagine any sane business would risk putting any amount of development time investment into their stuff.
You know appengine has been around for 8 years right?
Same number of years Google Reader had been around when it was sunset.
I bet it's earned Google infinitely more dollars than Google Reader!
What happened once it (GAE) reaches 9 year? gasp ...
Does Google appear bored with this? Long before it was even a product, this kind of service was a very core competitive advantage that Google has cultivated, and the much larger competitive landscape means we're looking at a product category of the size & scope that would make sense for Google to tackle.
It's heavily used inside Google. Tons of google-internal tools are built on App Engine.