1) the untapped market potential is still huge (I myself don't own an Android or an iPhone - although I'll be buying an Android phone soon)
2) The customers gained by Android and iOS are not as loyal or as committed to each platform as the early adopters of the home PC were. When you bought your first PC, you usually spent a lot of money on it, invested in software...etc. By contrast, most smartphone buyers just need a phone with an internet connection, decent browser and a Facebook app. Other apps and games are just a short-term bonus that quickly become a bore. Also phones are cheap or free with a contract (in North America at least) so switching after your 2-3 year prison sentence with one of the carriers is not a huge leap of faith.
Having said all this I personally hope Apple and Microsoft lose badly and Android wins. Purely based on principle and to a lesser degree my own mild case of fanboyism.