You use the term Android like it is a corporation or a brand. Are you comparing iOS to Android OS or Apple to Samsung, Google etc.? I agree that Apple commands a relatively small share of the US mobile ecosystem, but where do the competitors stand?
But unless you can demonstrate that it sucks because Apple is doing something which qualifies as restraint-of-trade, which I would suggest is obviously not the case, that doesn't matter.
Theoretical competition is not sufficient to demonstrate absence of a monopoly.
The doctrine that it's your fault if your competitors suck makes no sense. It's weaponized tall-poppy syndrome.
The doctrine is that you can't exercise monopoly power in certain ways. Monopoly power is an empirical question, and does not turn on merely whether it is possible to describe a market in which another product exists, but whether that is a real market in which the products are in fact competitive.
But even if you have monopoly power, if you aren't illegally exercising it, you aren't in trouble. So you aren't punished for being an empirical monopoly.
iOS is not a monopoly since at least one other major operating system exists, with significant market share. (Whether Linux is or isn't a competitor is irrelevant.)
A monopoly by itself is not a problem. Only behavior ancillary to that monopoly is. But to get there you have you have a monopoly. I don't see how you make the case. Clearly consumers have choice.
Now, there's a case to be made for bad behavior, but its weak. Apple will argue that consumers have choices.
But I am not a lawyer, so I'll leave it up to the lawyers on both sides to earn some fees discussing it.
So, now let's introduce iOS into the equation. Apple can differentiate their product, but how much is considered acceptable before regulators complain? The DOJ was quite straightforward today, accusing Apple of using iMessage to degrade user experiences through exclusion. If it walks like a duck, talks like a duck, it's probably a...?