I think it would have been important enough for Google to be worth fighting no matter the cost, since it would have been the only viable way of replacing Facebo

I think it would have been important enough for Google to be worth fighting no matter the cost, since it would have been the only viable way of replacing Facebook. The user would be consenting, initiating and running the procedure on his own machine, so there's a plausible argument that it's fine. Regarding the antitrust argument, you could also argue that Facebook is a monopoly in social networks and that preventing such a data transfer is anticompetitive.