It's not illegal to just be a monopoly but it can be illegal for a company with a dominant position (Apple's iOS has over 60% share of the mobile OS market in the U.S.) to engage in anticompetitive behavior such as restricting choice and competition in areas like app access, payment systems, browser engines, etc. A "remedy" could end up being similar to what happened in the EU.
Short of Congress implementing their own version of the DMA, I'm not convinced there's a coherent legal argument to force Apple to open up, much as I am certain there are many people here who would be absolutely trilled for Apple to. I suspect the lawsuit will end up running into the same problem that Epic did, which is that while there are people who may dislike and have issues with some things Apple does, what they are doing under current US law doesn't come off as being illegal.
First, it's not illegal to be a walled garden (the Epic trial established that).
Second, Apple is entitled to charge licensing fees for their platform, and if you say that 30% is abusive, then I say - abusive compared to what? It's been there since day 1, the fees aren't there simply to cover payment processing, and other companies like Nintendo and Steam do it as well.
Third, I don't see anything about anti-steering provisions, which was ironically the one part that Apple did lose on.
The thing about US antitrust law is that you have to prove harm to consumers (to my knowledge at least). And a lot of what Apple does can be argued as benefiting the end user in one way or another.
For example, Apple Pay means that merchants never get my payment information, yet people here would rather that NFC be opened up to banks who might then favour their own payment apps and opt not to support Apple Pay at all, in addition to monitoring what I do and selling my data for marketing?
Or would you rather favour Facebook being able to tracking what you do on your phone?
Or maybe you would prefer not making each game on a streaming game platform be an individual app so the Parental Controls on iPhone don't work as they have advertised for a decade?
Likewise, why should iMessage be forced to support RCS when SMS technically works? It's not like WhatsApp or telegram support SMS fallback either, so I find that an odd argument to make.
It just feels like the DOJ is hurling every single accusation that they can think of at Apple, regardless of merit, in the hopes that at least one sticks. At least that's just the way it seems to me.