If it did happen, it would reduce their monopoly power and be a net gain for advertisers and consumers.
It would force them to be more competitive with other social channels like Pinterest and Snap. Some likely improvements would be lower (more competitive) auction prices and better advertiser support. Likely shift some power back to advertisers, particularly on the social justice and ethical leadership front.
Really, the only losing party in this scenario would be Facebook shareholders. But they have gained so much at everyone else’s expense over the past decade, so I’m not going to shed a tear for them.