I think Apple has four problems.
1 - Communications Network Externality
Any time you have a communications product there is a need to interconnect with other providers in order for the product to provide value. That's why we have mandatory phone interconnection and the world needs a standard for email. If an incumbent, let's say VZ, was able to say "we will add static to any calls that go to T-Mo" they have an unfair advantage because users who choose an alternate network from a new entrant are punished not because the new entrant is inferior but because the incumbent is leveraging the power of their network. It serves to forestall the development of competition.
iMessage is a communications product that is intentionally harming other network operators. I think this is a legitimate claim.
2 - Agreement to split markets
In business, there are always decisions to outsource and focus but sometimes it reaches the level of agreeing to divide up a market. Ford buying transmissions from a company that specializes in transmissions isn't an issue. But Ford calling GM and saying "we make trucks, you make cars" is illegal.
In the Google antitrust case, it came to light that part of Google's arrangement with Apple for the iPhone search deal was that apple had to stand down their efforts to develop a search engine. That is, in my view, an agreement to divide up a market. Google may make a unilateral decision to change its terms if Apple is seen as an emerging competitor but they cannot make an agreement to stay out of each other's markets.
A similar agreement that Apple must stand down AI development as part of a deal with Google would also be very suspect.
3 - App Store Utility
At a certain point, a platform reaches the status of being a utility where it is not practically possible to be in market without paying that utility. I could make widgets without paying the power company if I built my own power plant, but practically speaking that's not a competitive option. So I have to use the power company.
When something reaches utility status society has a need to create rules on how much rent that utility provider can extract and under what terms. The power company can't say "You owe me 30% of the revenue you get from your widget sales." They also can't say "You can't use the power from my nuclear plant to build solar panels."
The Apple app store may have reached utility status. It is not possible to develop virtually any software and many retail services without relying on the App Store. If so, they can't demand 30% of the retail proceeds from others products nor can they say that companies (e.g., Epic or Kindle) can't set up their own stores within their apps.
4 - Retail price fixing
It is illegal for a supplier to dictate to a retailer the retail price of a product. They can set MSRPs, provide co-marketing funds, etc but they cannot say "The price of a Ford is $1000" and require everyone to do that. A big reason that Apple can get $1400 for an iphone is that behind the scenes they unleash enormous pressure on the carriers to do "free" iphone deals. It could be argued that this pressure reaches the point of price fixing and market manipulation.
These topics are intertwined because, collectively, they build on one another in a way that harms competition in multiple arenas.
The nature of anti-trust is almost always a judgement call between not punishing success and recognizing when someone has unreasonable market power. If I were the DOJ, I would go after them in this order. I think this case should be used as the basis to update antitrust law for the digital age.
My $0.02.
