Firebase Team AMAView other answers to this thread
Is there any plan that Firebase RTDB will be replaced by Firestore?
Oh, you probably wanted more info, didn't you?
Okay… no, I think there will be situations where RTDB will always be a better solution. The Realtime Database is optimized for situations where you want small chunks of data that's updated frequently (perhaps to power a poker game or something), where as Cloud Firestore is optimized for larger chunks of data that are updated less frequently. (Like, say, a blog post.)
I'm not sure if there will ever be a day where one database can serve both or your needs, and we have lots of customers happily using each one of them, so I think we're happy to continue running two NoSQL databases for the foreseeable future.