I don't seeing Android being like Symbian, but I do see a fall. And it's not entirely the fault of the Android OS.
Samsung is the BIG Android OEM right now. If they go with Tizen, many users will follow. That ends up almost killing Android on the high end.
Sure, there will still be HTC (if they survive long enough), LG, and whoever is left trying to take Samsung's spot, but there may not be a spot to take. Samsung's branding is so strong. It's stronger than Android's branding. Just like how Lumia's branding is stronger than Windows Phone's branding.
If Samsung goes with Tizen in 2015, I could see the end of 2015 looking like...
Android | iOS | Windows Phone | Tizen | Other |
25% | 35% | 20% | 15% | 5% |