What if FB only maintained their Android mobile app and made iOS second class, gradually lagging in features and operational support/stability?
If that experiment 'succeeds', Instagram could follow.
What do you suppose would be the outcome?
The problem is the iOS demographic often correlates with one of the most valuable to advertise to. While targeting will become harder again, which might hurt prices, overall its not like that ad spend is going to go away. Targeting will become more coarse-grained, but at the end of the day, facebook has so many on platform data sources, that it remains quite profitable to serve them.
Of course I wish a shift did happen, but driven by consumer values (right to repair, appstore policies) to a more open platform.