"Google will announce if we intend to discontinue or make backwards incompatible changes to this API or Service. We will use commercially reasonable efforts to continue to operate that Service without these changes until the later of: (i) one year after the announcement or (ii) April 20, 2015"
So you have a year, at a minimum.
"Unless otherwise noted by Google, material changes to the Agreement will become effective 90 days after they are posted, except if the changes apply to new functionality in which case they will be effective immediately. If Customer does not agree to the revised Agreement, please stop using the Service."
Even if App Engine isn't tremendously valuable now, it's a growth market, and easily monetizable. Google isn't going to burn a bunch of customers who could be - if they end up being the next Netflix - worth hundreds of thousands of dollars in recurring revenue.
I wish people would stop with the hyperbolic Google Reader stuff. Google's bread and butter are web ads, and they're a shrinking market. As Facebook can attest, mobile is hard to monetize, and it makes a lot of sense to focus on areas where people can just pay you directly. AWS has already validated this market, Google has the capacity to deliver, they've just fumbled the execution wildly.
That kind of cynical thought should be eliminated.
Not that many.
>and Google has probably invested tens of billions and thousands of engineers into GAE
If we're speculating, I'd say, far far less.
Except for the parts they use themselves too, anyway.
Stop wallowing over reader and move on.