which mobile OS will most likely to dethrone Android?

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http://androidforums.com – Android currently have 75% of the smartphone market share, undisputed in terms of number of users and popularity, but one day, as with everything, nothing last forever. Do you think it will be one of the currently available mobile OS that will dethrone Android in the future or will it be something completely new that does not exist now. Firefox OS, WP, Tizen, Ubuntu, or some other linux OS? I think and hope Android will be around for a long time like Windows for desktop. (General)