alt Hacker NewsThe dominance benchmark is usually around 70% market share. Apple's iOS holds a 52% to 58% market share in the United States. It's not terribly far off tbh.
More to the point, the EU have already slapped them with two separate $500m fines having repeatedly found that Apple used iOS and the App Store to break antitrust laws.
The US have a similar case hinging on all sorts of things ranging from green text message bubbles to heavily restricting access to the NFC API and affording to itself the privilege of host-card emulation via Apple Wallet.