Okay, let's say that's how it plays out.
Advertisers still measure ROAS.
There's no way you can block me from tracking my ad that sends traffic to my site. I just need a custom URL for each ad or campaign, and that's dead easy and tracking blockers can't mess with that because that's baked into the way the web works. So I can always get last click attribution. I can even get more sophisticated attribution modeling because every time you come to my site, I'm setting a first-party cookie and most people are not blocking those.
All that to say that advertisers of even the least sophistication will still be able to calculate ROAS. So at that point, they adjust their bids down. If Facebook has enough eyeballs to sell, everything shakes out to a new equilibrium where we are now, but you need a lot more eyeballs, and that does not seem the way FB is headed. So it takes so many ads to make a sale that there are not enough eyeballs. Bids go down. Facebook revenue goes down and advertisers get crowded out.
Does that sound right? If so, what then?