There is no alternative.
Apple isn't going to let them build on top of iOS, and anything except those two is dead in the water because it'll never have users because it is missing a bunch of critical apps, and will never have those apps because it doesn't have users.
> There is no alternative.
There have been several projects like Ubuntu Touch to create an open Linux for smartphones.
That would be an open alternative.
Android is not open.
Remember when we thought IE's monopoly could never be broken? Or windows?
Anything can and will go down. Nothing is forever.