An iphone seperated from the mac business would either need to run it's own icloud that is open to other companies (e.g. mac if it still wants to sync with mac) or it needs to use 3rd party integrations (e.g. icloud if it's its own company or still part of mac). In one case iphone is making more money and is more open in the other it's spending more money and is more open (which is which; who knows) in either case mac and iphones incentives are no longer aligned on keeping a closed icloud and one (or both) of the products would likely become more open which is good for customers and the whole point of anti-trust.

The sucess of iPhone/mac/icloud doesn't really factor into it (some of them will shrink and some will do better as they're not subsidizing other parts of Apple or can canibalize other parts of Apple's market). You'd be right in asserting the success of Apple is a factor, as those who are arguing for such a seperation would say, it's making outsized profits by not competing fairly in several of the markets it's in.