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.