Since I am not using the Google Cloud Platform App engine service anymore, I disabled it and got the following error on Firestore:
The workaround I could find is just to deploy a dummy app engine fully managed application. Still I think this should not be the right way of doing things.
Is there another way? Why did Google choose to link Firestore to App Engine?
firebaser here
Cloud Firestore is an evolution of Cloud Datastore, which in turn is the (originally unnamed) database in App Engine.
So it's not as much that Firebase chose to link Firestore to App Engine, as it is that we can't unlink it from App Engine without rewriting a lot of infrastructure. That may happen at some point, but at the moment you'll need to keep App Engine enabled to use Firestore.
By the way: this is also the reason that for a long time you could set a spending limit on Firestore, but not on other Firebase products: it inherited its spending limit from its App Engine lineage. When the ability to set a spending limit disappeared from App Engine, Firestore also lost that ability.
A lot of Google Cloud products are actually linked to App Engine deep down inside cloud projects. I'm not sure anyone is going to be able to give you a satisfactory explanation of why this is (unless it's coming from an engineer from Google). But I can tell you that it's a long-standing legacy of the way cloud projects work. App Engine is the original Google Cloud product, and much cloud infrastructure was built around it. That's all implementation details, and you don't need to know why that is - all you have to do is enable the App Engine API, even you don't use it directly.
