I think the argument is that more affluent users would be more likely to opt-out...

I think the argument is that more affluent users would be more likely to opt-out, leaving behind less affluent, and therefore less-valuable users, making advertisers desire less and less to advertise at all.

Facebook would be faced with trying to get as much revenue out of the 5% of their users who pay as out of the currently-100% of users who don't, and that would be a very hard thing to do.