I’m sorry to break this to you, but Remote API is around for a while. Since 2012 at least. Not sure when was it exactly introduced though. (According to the javadoc since 2010.) This is why I really can’t grasp what you are feeling about GAE if you missed such a core tool. It’s like not knowing you can connect to SQL servers with GUI tools.
Yes, I read what you wrote, but writing apps as platform-independent rarely works/worth it, and there is a reason for that. We started by using the native entities as well, but realized soon enough how powerful objectify is, and started using it extensively. Definitely worth it.
Anyway lets get the facts straight. There were missing features when you started using AppEngine — like proper analysis of data costs, something I assume you were aware of at that time —, there were core feature(s) — remote api — you missed and you refused to improve your code to address your issues — for example by using objectify — , and that is why you left/trying to leave GAE?