In 2011 microsoft were deciding whether to go with Android. They chose not to, and eventually bought, raped and murdered Nokia. They are now completely irrelevant in mobile.
There are two fascinating what-ifs around this. First, had Nokia been able to resist them, and release their own Android phone, we'd not see the massive Samsung Android dominance we have today, but likely a Nokia/Android equal split with both more than capable of innovating and delivering the quality needed to keep ahead.
The other one is Symbian. Don't laugh - this is 2011 days. Given that microsoft decided to go their own way anyway, what would have happened if they had done this but instead used the massive engineering resource they just bought to further develop Symbian to the point it had the same iOS/Android features? Would we today see the duopoly, with microsofts own at < 1%, or would "Microsoft Symbian" be a realistic 3rd contender with all the mindset, ecosystem leverage and brand loyalty that came with it, if handled right?