Thanks for your answer, I think that at the time of ReasonML presentation and a couple of other times I had the impression that there was quite of a push on that direction, but I may be wrong.
Additionally, the opinions of popular leads, managers etc on what should be used will count more. Winning their mindshare will win you a lot of FB org.
Winning a lot of FB org will exert a tremendous influence on the Open Source community as you now have large numbers of engineers working on open source projects on premium pay.
I believe this is how it works in most modern large engineering orgs. After reaching a certain critical mass of adoption, a team will be hard-pressed to use anything else, especially if reusing other team's components requires you to master, say, Redux anyway.