That is a really naive view of market share. The market for phones is tiered, and Apple dominates the tiers it wants: The profitable ones. It doesn't take 51% market saturation to have monopoly control.
Anybody who wants to get out of Apple's system can easily pick up a different high-end phone[0], with different tradeoffs, with few repercussions beyond how their Apple Watch (if they have one) works and the color of their messages when texting iPhone users.
That's not monopoly power.
[0] as a consumer I care about the phone's features, not its profit margin for the manufacturer. There's still a healthy ecosystem of non-Apple companies making phones.
