That is not true. there are many things corporations cannot do even if they want to. There's lots of different regulations that impose restrictions and obligations on companies against "what they want".
So now it's just defining where that line is, which is what law-making is.
This is like saying a state-mandated ISP/Gas company wouldn't be a monopoly since you can just move to another country.
You might wanna push the argument that nobody is forcing you to use iOS, but I think the problem runs deeper. In many fields and even just everyday life, you are now required to run a modern phone OS (eg. the option for not having the official COVID green pass app here in Italy is to find an authorized place that will print you a green pass on paper, and it's hard to find one, let alone schedule everything and get there without a pre-existing pass) and we're barely lucky that Google is playing by laxer rules than Apple is. If not punishing existing bad actors, these rules are a nice framework to prevent them for taking over in the future.
Apple's pseudomonopoly does not strike me as the root cause issue here, developing processes and using tech to solve a people problem does.
Also, it's the country's duty to protect citizens from themselves using laws and regulations. For an example of what kind of "far west" digital tech is living right now, I point you back to before TV was regulated as heavily as today, where companies used cartoons to advertise smoking cigarettes to children (https://www.youtube.com/watch?v=NAExoSozc2c).
No, they can't do whatever they want with it. Consumer protection laws are an obvious example for how corporate interests can be limited.
> because it's far from a monopoly since there are alternatives.
It's a duopoly. iOS and Android together represent about 98% of mobile devices.
