Firebase Team AMA
View other answers to this threadDeveloper Advocate at Bearer.sh
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.