Let’s take that to the next level.
Microsoft couldn’t sell third party software that integrates with Office. GCP and AWS couldn’t run marketplaces that sold services from third party companies like ElasticSearch and Mongo labs. Who would be hurt worse if AWS stop selling ElasticCo and Mongolabs hosted services and just sold their own ElasticSearch service and Mongo compatible DocumentDB?
Walmart couldn’t sell Walmart branded goods. CVS couldn’t sell cheaper versions of name brand drugs, restaurants couldn’t sell name brand sodas and their own food.
How far do you want to take it?
Seeing that if Netflix couldn’t create their own content and had to buy content from third parties do you think that would help Netflix more or give other studios even more leverage to raise their prices?
What if Apple said fine - we can’t offer Apple Music and Spotify so we will just kick Spotify off the platform and not allow third party games and only allow games through Apple Arcade.
They could also kick off Office and force everyone to use iWork.
But why stop there? Let’s also make a law that says Apple can’t sell third party accessories and Apple products in their physical Apple store and Google can’t sell Pixel devices along with third party phones on their website.