We already have a 3rd mobile OS, it is called Windows Phone and is doing quite well (actually more than iOS) in a few European countries.
Windows Phone are the only alternative to Android, when you are buying pre-paid phones and not willing to shell out more then 500 euros for a mobile phone, when earning 400 euros on average.
Even the contracts are not an option for such users.
I can certainly understand the price issue, it's the reason I went with a WP here in Finland: My Lumia 1320 was only 300€. Nothing comparable for Android or iOS was available for less than twice that.