Android is a very well done open source OS for mobile systems. Its the best in the world at what it offers by a long shot. Will it forever remain the best in the world? Not likely, things change and it all depends upon execution. Tizen may take years to get to a point where some of android's OEM"s can look to offer a wider range of phones with Tizen. That may happen 5 years down the road or may take longer.
It's actually a terrible useless, watered down open source OS. if you want to make it usable you need the Google API's, which are closed source.