[JeremyNT]

The long and short of it: he bet everything on Windows Phone.

I think you can envision a successful strategy that would have included Windows Phone, potentially even as the only platform they offered, but Elop killed off everything else in a very visible way before WP was really ready. When it became the only offering, they alienated existing customers and left them with no obvious migration path.

WP sales have started to recover, but IIRC they're still lagging behind what Symbian was doing at the time Elop took over. Conceptually WP may have been a reasonable long term choice, but there were certainly ways to get there that didn't cost them so many existing customers so quickly.