At this point there are only two mobile phone platforms with apps. iOS is smaller, but has tons of apps. It started the trend and people seem to like to pay more money on iOS for apps than they do on Android (I don't know if that's as true today as it used to be, but I think it still holds). Even if users don't like paying as much on android or paying it all, Android has a larger market share in thus is still worth developing for. Both platforms have hundreds of millions of users.

Windows phone is dead. BlackBerry is dead. Palm OS. The previous versions of Windows Phone. The Sidekick. Amazon Fire Phone. Canonical's phone, the Firefox phone, whatever other Linux phones I can't think of.

It doesn't matter how big the company backing it is, even if they actually do a good job (like supposedly the recent Windows Phones). At this point the first mover advantage for Android and iOS are too big. Even if you spend all the time to make a great platform no one's going to buy it because the top hundred or thousand apps aren't available. Because no one's gonna Byatt know it's gonna make apps for because there's no one to buy the apps. You're stuck in a cycle.

Microsoft tried to buy their way out of it, and blackberry did too to a degree. But it doesn't matter if you pay to get the top 40 apps from iOS, because nobody uses all of them. They use some percentage of the top hundred apps. Or 500 apps. So every users going to have a couple of apps that they really want that are unavailable and will never get ported. The apps that do exist (like Twitter and others) often don't get updates, and if a game or something does come out for the platform it's not until years after the big two.

Until something very different comes along, or some HUGE external force appears, we're only going to have two big phone OSes. The only thing that slightly possible (in my eyes) would be someone as big as Samsung pushing their own OS in other countries and possibly getting a good foothold there. Maybe Xaiomi.