Apple is what, less than 20% of phone sales? So it's hard to see how that constitutes a monopoly. And if banning 3rd party apps is enough for a lawsuit, then why doesn't that apply to Microsoft, Sony, and Nintendo for their game consoles? Why doesn't it apply to Amazon's Fire tablets, or Kindles, or Huawei phones, or Oculus headsets? All of those devices have similar restrictions.
Unless customers are coerced or misled, or returns/refunds are difficult, I don't see the need for government intervention. Apple's software restrictions hurt the iPhone's market share. The same goes for charging high fees for app purchases. Customers and developers can (and often do) choose other devices for being less restrictive about what software can be run on them. If an informed adult chooses a locked down platform because they prioritize other features, why should the government stop them?
I can see an argument for requiring labeling (similar to warnings on cigarettes), but a total ban seems like overreach.