The analogy doesn't really work because cryptocurrencies aren't interchangeable the way phones are. A better analogy would be if each phone could only talk to people using the same phone brand. If everyone is on iphone and you need an iphone to talk to anyone you care about, the iphone becomes more valuable because of its scarcity. The network effects of bitcoin and the lack of interchangeability between different cryptocurrencies helps secure bitcoin's spot as the leader.