Because that argument worked so well for Microsoft in their antitrust suits when they locked down web browsers to just Internet Explorer right? "Just go to the other Duopoly, stop criticizing fruit computer company!"
That is the complete opposite of what it means. Theodore Roosevelt lead trustbusting efforts on the railroad monopolies for a reason, as when certain players get so big it stiffles innovation as any semblance of other companies attempting to grow. We already have a real world example of what happens when there's no regulation, it was called Trickle Down Economics, AKA Reaganomics in the 80s. The idea being that if you let the major players do whatever they want and get bigger that it benefits everyone.
A sound idea in theory, but in practice as we learned from Reaganomics it doesn't work. All Trickle Down Economics did was make the rich more powerful while the lower and middle class got nothing in return.
Without any kind of regulation you get a cyberpunk dystopia. Just look at South Korea, where Samsung has market dominance over pretty much every industry there, even having seats in the government.
We're not talking phone manufacturers, but mobile operating systems. In the early days of smartphones we had competition. Besides iOS and Android there was Windows Phone and FireOS, both of which were unfortunately shortlived due to the fact they did not have the app libraries that iOS and Android had with how late to market they were, and both costed Microsoft and Amazon billions. Your only choice for a mobile OS is either iOS on iPhone, or a different flavor of Android. That's not competition, that's a duopoly. Either be at the mercy of Apple with their OS app distribution guidelines, or at the mercy of Google's.
See again above the fallacy of trickle down economics.