>Apple's opt-out by default data policies cost Meta billions of dollars the quarter it was introduced in the OS update. Meta would assuredly restrict their apps to a Meta store to get back those billions.

This conclusion doesn't follow from the premise. Look at Meta's revenue since the roll out of ATT. They're essentially back to where they were. They've figured out ML based probability targeting. If anything, they're in a better position than they were pre-ATT, because now they really have some secret sauce in there, because no one else seems to have been able to figure it out.