Firstly the video, though I've got the relevant quote below too.
From the description:
We are tremendously honored to have Bill Gates among our small group of luminary LPs whose financial capital and engagement power the next generation of Village Global founders and teams.
We asked Bill to join us in San Francisco for a fireside chat on the future of technology, early-stage investing, and the nature of innovation and impact last November. Bill’s passion for the next generation of world-changing entrepreneurs inspires our network of founders to succeed.
(Village Global is an early-stage venture capital firm backed by some of the world's most successful entrepreneurs including Jeff Bezos, Mark Zuckerberg, Reid Hoffman, and many more.)
Gates said (of Mobile):
"You know, in the software world, in particular for platforms, these are winner-take-all markets. So, you know, the greatest mistake ever is the whatever mismanagement I engaged in that caused Microsoft not to be what Android is. Android is the standard non-Apple phone form platform. That was a natural thing for Microsoft to win. It really is winner take all. If you're there with half as many apps or 90% as many apps, you're on your way to complete doom. There's room for exactly one non-Apple operating system..."
Although the obvious comparison is Windows Phone to Android, that's not actually accurate. Windows Phone was, by Gates own admission above, too late to market - Android was already established as the main iOS competitor by 2010 when Windows Phone 7 arrived, and by 2012 when Windows Phone was effectively fully realised, in 8.1, Android was dominant worldwide.
However, Windows Mobile (it went under several name changes, but let's use that) was very much a thing between about 1998 and 2004, fully touch-enabled, with phone stack, integrated data, and more. In many ways it was ahead of Symbian, which became the dominant smartphone OS from 2004 to 2010. So why did Windows Mobile, at least half a decade before the iPhone and Android, not take off and rule the world? Lack of investment from Microsoft. And that is most definitely Gates' fault, he can't blame Ballmer for this. Windows Mobile phones were seen as a niche product and not the utterly dominant computing form factor that smartphones are today. With even modest development and investment, Windows Mobile need not have died out - the rebirth as Windows Phone could have easily happened organically rather than having a half decade 'drop out'.
What might have been. Again.