It's a duopoly, but in terms of revenue/prestige it's a de-facto monopoly. The simple fact is that that Apple can bully because NO developer can afford to walk away from the Apple App store; and the App Store is simply too big for the market to self-regulate.
The anti-trust cases against Microsoft ultimately allowed Firefox, Opera, Chrome to flourish and eventually lead to the demise of IE's monopoly position.
Did it? Microsoft didn't get pwned in court over the fact that the OS came with a web browser, they got pwned in court because they were enforcing deals on OEMs that didn't allow them to include another browser.
I'd argue that Firefox, Chrome, and Opera did not flourish because of a court ruling, they did so because they were better in every sense than IE. Microsoft has never stopped anyone from downloading whatever browser they choose.
The anti-trust cases against Microsoft ultimately allowed Firefox, Opera, Chrome to flourish and eventually lead to the demise of IE's monopoly position.