So by that argument if MS would have disallowed installation of any other browser it would have been OK, because competitor could just build their own operating

So by that argument if MS would have disallowed installation of any other browser it would have been OK, because competitor could just build their own operating system. The whole argument is that Apple is leveraging the position in one market (hardware) to gain advantage in another market. Your answer is "competitors can always build their own hardware" don't you see that this is a nonsensical argument?