So far me and 215 people have asked for adding react-native-firebase to expo. We have been the most trending FR on canny until last week!
I think I know why we haven't heard back from Expo and here is my solution to your concern.
Currently, the highest voted feature request is for "Background location tracking" with 380 votes.
Subtracting 216 from 380 leaves us with 164 people who are either neutral or against adding Firebase support.
That is understandable because nobody wants extra code bundled in their app.
Making different flavors of Expo (like Linux) is going to solve this problem.
I think, flavors should focus on three main functionality:
a) Navigation: React-Native-Navigation or React-Navigation
b) Backend: React-Native-Firebase or Appsync
c) Localization: React-Native-Maps or Mapbox-GL
In my opinion, Expo should see itself as a very experienced react-native developer who happens to be a framework!
In React-Native ecosystem, there are many out dated libraries that sometimes don't work well with other libraries.
Only experienced developers know about that.
Based on my observation, Facebook has decided to rely on the community. So somebody has to bring it all under one umbrella, and that could be Expo!
But in order for this to work, Expo has to set some high level guidelines, so developers follow them and build native libraries that work well with Expo's flavors!
I must say that I really appreciate the hard work done by the folks at Expo. You might have already thought about this. I am just stating my opinion here and that's all!