You can split them up if they become so successful that uncompetitive behavior just falls out of it. Like people who want to split away the app store from the iPhone (regardless of the fact that other phones and app stores exist: Apple's vertical closed ecosystem makes the iPhone more popular).

More popular than what? Android is 70% global market share.

Apple had a 75% market share of phones that cost $600 or more in 2022...

If you define your market small enough anything can be a monopoly. I bet Apple has a 95% market share on phones that cost more than $1500. I bet Google has 75% market share on phones less than $100. So what?

We are talking about the App Store here, correct?

Do you think that for an App Store attached to a phone whether the phone cost $100 or $1,000 matters?

Because everyone related to those App Stores does.

Apple's control of the app store makes the $1,000 phone more appealing. If they opened up the iPhone to other random app makers, the phone would be less appealing and they wouldn't sell as much.

Profit share.

You don’t need a monopoly to be anti-competitive and get broken up because of it.

Alcoa aluminum is a counter example but that was in a different regulatory climate.

Laws are laws. That they aren't enforced sometimes is a pity, but that doesn't negate their existence. The Sherman Antitrust Act is pretty broad, and even simply "dividing markets" can be a cause for action.

For those that dislike these laws so much they would rather pretend they don't exist, I wish they would spend some time reading and understanding the context under which these laws were passed. These types of arrangements, when allowed to balloon out of proportion unchecked, cause massive amounts of damage to the average american citizen and their individual and collective long term interests.

Lassez Faire style regulation was effectively tried and it was an unmitigated disaster for the citizen, who is, and should be, protected in our Constitution while our corporations must obviously be constrained to subordination by it.