Comment by microtonal

But without the deep app ecosystem backing them up,

But there was also a genuine sense of excitement about Windows Phone 7, they had some single-digit marketshare, the other ecosystems were not as big as they are now, and Android was pretty bad at the time.

I think one of their fatal mistakes was the Windows Phone 7 -> Windows Phone 8 transition where they left many early adopters out in the cold (IIRC none of the WP7 phones could run WP8).

I agree that building an ecosystem was a large part of the problem, but Microsoft also messed up royally by making a hard cut in the platform when it was just starting to take off.