Ackchyually you don't need to be a monopoly to warp the whole market in your favor.
Windows was a monopoly back in the day with ~95% market share, but users could easily install any other apps and any other browser besides IE. The issue was many users stuck with IE because it was the default, but Microsoft wasn't blocking them from installing other browsers than IE or other apps not sold through their channels.
Meanwhile, Apple might have only 57%-60% market share instead of ~95%, but unlike Microsoft, it's actively blocking users form installing any other browser that's not Safari based and any app that's not on the AppStore, therefore doing even more damage than Microsoft was doing back in the days.
And speaking of iPhone market share, despite being it 57%-60% of the US right now, it's also the most wealthy and influential of the market, and looking at the demographics, it's at 87% amongst US teens, so it's safe to assume those teens will stick to iPhone when they grow up due to inertia and network effects, and then iOS market share will be close to 90%, therefore a guaranteed monopoly.