That's not really the problem.
Suppose there were two cellphone platforms and they were both equally terrible, or equally good. But each one locked the customers out of their own property -- you have to use their app store, their cloud services, you can't install their operating system on anyone else's hardware or vice versa. Mandatory lock-in to all of their other services, competition impaired.
This is not okay just because there are two of them instead of one, because it's what causes there to be two of them instead of dozens. You can't enter the market without replicating the entire vertically integrated conglomerate because someone who makes only hardware, or an operating system, or an app store, can't use it with the existing ones.
It removes consumer choice, and each of the conglomerates could continue extending the stack in either direction until you have to buy your house and car and all of your groceries, appliances and media from the same company as you get your phone.
This is not the same thing as vertical integration. The same company could make a CPU and an app store without having them bound together so that you can't choose one without the other. You could choose to get everything from the same company, but you shouldn't be required to just because that's what the company wants.