As for why the platform has been all but dropped? The executive boils it down to one main reason: the difficulty of getting developers to write apps. Microsoft tried paying companies to produce apps and even wrote them itself when creators couldn't or wouldn't get involved, but the number of users was "too low for most companies to invest." Why build an app for a relatively small bunch of Windows phone owners when there are many more Android and iOS users? Belfiore himself switched to Android for the "app/[hardware] diversity."
It's a bit more complicated than that, of course. You can point to a few other factors in Windows' fate on phones, such as slowness in responding to Apple and Google as well as an inconsistent hardware strategy (you could rarely count on getting a timely sequel to a handset you liked). Whatever the reason, it's safe to say that Microsoft isn't just acknowledging that Android and iOS hold a clear lead -- it's quashing any hopes for a comeback, at least for the foreseeable future. That's no doubt sad if you're a long-time fan, but you'll at least have software like the Microsoft Launcher and Edge to keep you company.