this is a historical fact. Did it survive? No. I hope you're happy with this answer...
iOS had no competition, because Symbian had no smartphone apps. It could run those old button-nokia kind of apps, but they didn't make any sense on a touchscreen. Other than that, their app store was pretty much deserted. Just like Windows Mobile.
Android had no competition, because iOS was completely locked down and could only run on Crapple's hardware. People wanted non-overpriced hardware, and that's where Android came in. No competition here. It wasn't competing with iOS because it wouldn't even run on the same hardware.
Windows Phone already had Android on its tail, so it had to do something really great in the very beginning to shake it off. There just can't be two mainstream OSs at the same time, as shown by desktop computers: there was DOS, OS/2, MAC OS, and whatever, but the only one that remains is Windows (pretty much updated DOS, if we talk in very broad terms). Mac OS too, but once again, it's not a "mainstream" OS since it only runs on one OEM's laptops.
Windows RT was in the same situation. Android was ruling the tablet market, iOS being once again a separate talk, and they just couldn't co-exist. And then x86 tablets appeared. There is no competing with that, you know.
There is also Linux, but it always was and still remains a niche OS. It doesn't compete on the number of apps, since it's not intended for general audience. The people who use it write their apps themselves. So, it remains a niche OS.
The point is, two mainstream OSs just can't co-exist. One must die. And a new OS with no apps taking down an old, widespread OS used by everyone, everywhere, having an app for everything, without any huge downsides? Fat chance.