This question/concern keeps popping up every few months for years now. Being a very early GAE user, I have my opinions and history based analysis that I have posted before. Recent signals, having attended Next as well, make me more optimistic than usual.
Today I want to add a new perspective: The question really is how much risk do you want to take? Recently the US stock market listed Snap. When you get listed in the US you are required to provide the investors an S-1 that contains all the risks of investing in this offering. Open the Snap S-1
here and search for Google and read these paragraphs. Bottom line, Snap says "we are alive as long as Google Cloud is alive". Then look at this Quora thread:
https://www.quora.com/How-does-Snapchat-use-Google-App-Engine Bottom line between those two, for now Snap, a US public company, is alive as long as GAE Standard is alive.
So, I conclude that Google will be committed to GAE Std. for a while. These are the kind of signals to look for; "formal statements" are good for few days, and we have seen many of those over the years, but can come and go as fast as an e-mail can be written. Does anybody else know of other such "too big to fail" successes built on GAE Std? The more of those the more confident I would be...
Another signal of course is where Google puts its investment dollars. In the early 2010s timeframe Google puts all its (few at the time) eggs in GAE. The marketplace did not reward this approach so Google increased its Cloud investment and shifted all of it to build a better AWS because this is what the volume enterprise and startup market seemed to want. However GAE survived and remains a competitive advantage and differentiator for GCP, so I hope Google delivers and puts back more investment to GAE now.
PK
PS If the two GAEs merge downstream I am fine as long as I do not have to revisit (ie modify/test etc) code in production that has been working for years. The more of that I would have to do the more negative I would become of such a development.