I get this, but this is what bothers me sometimes with these laws, because getting all these games for $10 is better for the consumer, that is why it dominates the market.
Breaking it up just means you end up with a worst product for the consumer and a higher expense.
I think you need to have a longer time horizon. Microsoft cannot justify spending $70B to offer the entire catalog of games for $10/month. They'll use this economic advantage to muscle out the competition and then they'll start adding tiers, raising prices, and other anti-consumer behavior.
This is a common tactic to win public approval for anti-consumer acquisitions. It's always better for there to be more competition, not less.
Breaking it up just means you end up with a worst product for the consumer and a higher expense.