First at all Congratulations, thanks for your contribution and very good luck Guido.
We have to be honest, it is a big kick to the google app engine project because it is not the PaaS most used by developers yet.
One year ago Yukihiro Matsumoto was hired by
Heroku and if
Engine Yard had hired Matz, certainly the good numbers (developers' quantity) had been better for Engine Yard.
I am sure that in the past a lot of developers moved to App Engine with Python because it had the support of the creator of that language and of course a Big company too, call it "Commercial excuse for choose a platform". These excuses exist and people who take decision in companies use them.
Google now has to move the pieces smartly by finding a new marketing strategy and in this case Google Ads doesn't work, because in this kind of products where the number of users (Developers in this case) is very important and there are not many excuses for use it now, only that is a product supported by a big Company (Google) but Heroku, Azure, Engine Yard and other could have more reasons and they will take advantage of this.
Don't get me wrong, Guido made a gorgeous contribution to App Engine but he is also a marketing reason to attract developers, in spite of there are a ton of Python talent in App Engine team.
I am still a fan of App Engine (reason because I built one of the most used App Engine
Boilerplate) and I hope this PaaS grow much more, hat off to Guido.