This isn't an opinion. State where they have been found to be a monopoly? To be a monopoly you have to be the only 1 (hence the mono in monopoly). They are not the only one. At best they are a duopoly when it comes to operating systems used on mobile phones. While they did not always used to be a duopoly in the OS space. They didn't do anything to force out competition in the space. Blackberry, Nokia, Palm, WindowsCE, Symbian, WebOS, etc. All failed on their own. And we still have plenty of hardware vendors out there competing with Apple's iPhone.
Google is open by default. Everything you want to do is possible with an Android OS. So Apple has competition in the space. And even though Apple has chosen to be a closed system. People still purchase their products. More in the US than the rest of the world. Knowing exactly what it is how it works and what you get for the money. If at any point you don't like it or don't agree with the "Apple" way of doing things. You can move away from it to another device and OS that will cover all the needs you may have or want. They both have the ability to help you migrate to and from each other with a USB cable. All your data goes across. You will have to get your apps again, but that is a small price to pay to be able to go between when you want.
No one is trapped. No one is forced. The door swings both ways.