No one is forcing you to buy a particular device.
True. But society in practice requires a smartphone with one of two operating systems to live a normal life without significant efficiency losses in your day. Now all phones with both of those will be completely walled off. You'll be forced to participate or make your life a lot less convenient.
Surely you wouldn't defend absolutely anything happening to say roads just because you're not forced to drive, technically speaking?
Furthermore, if you fundamentally allow this behaviour, the market forces are sure to push us to an end state where users simply have no control, and there are no viable alternatives. We are most of the way there already when it comes to smartphones. The cost of entry to this market (many $billions over many years, if you can even manage to gain meaningful marketshare at all), and the amount of money that is on the table (30% of the $billions transacted on a successful platform today, but who knows how far they push with a real stranglehold) means that it is virtually impossible for competition to solve this problem.