Daydream is going to be hard to beat. Google is poised to release standalone Daydream headsets soon. They control Android. How many people bought the "Facebook phone" compared to the many other Android phones available? I think Facebook is in far more trouble in mobile than you assume. There are no givens right now, but I do agree thats the arena they should be competing in.
My gut feeling is that Facebook VR will just end up being a Daydream app and a Facebook branded Daydream headset.
Google is ruining it in stupid ways. It has been deliberately made incompatible with all Cardboard apps. I get that Daydream is technically much better, but it has very little content, and there are tons of VR apps made for Cardboard.
Now my Daydream is gathering dust. I try to use it so rarely, that every time it I try, it refuses to run, because I have to install yet another controller firmware update. It's is a slow process and has about 75% chance of getting stuck and never finishing.