It wouldn't be that straightforward

Probably what OP is suggesting is that in return for that $5B, Apple will call up Google and say "hey we're adding these new privacy protections, will that be a problem?" and Google will have time in advance to get around them

Google has an easier time providing targeted ads via search keywords. Facebook users spend most of their time in their feeds which is a relatively weak signal for monetization.

Note that google appears to be acting as though they are effected, you can see this in their push for FLOC/topics (but the impetus for this could be coming from android moreso than search)