You mean Microsoft? No backwards-compatibility with Windows Mobile to begin with (so companies can't reuse their existing investment into line-of-business apps on actually nice modern devices either), then they reset the ecosystem 2 times (once during the WP7->WP8 transition, another time during the Windows 10 transition).
I was a telco product manager at the time and I can tell you right away that it wasn't developers that killed Windows Phone. This book (https://asokan.org/operation-elop/) tells part of the story, but the telcos I worked for (and competed with) definitely played a big role.
I did own a Treo and loved it up to the OG iPhone - I repaired the eff out of it in the hope that something worthy would come along. I kidded myself I would write apps for it. I'd previously played with Simbian tech (and met a very bitter Simbian team dev in London one "eXtreme Tuesday Club" meetup in 2003). I had a Psion Organizer way back and Palm pilot. I thought Palm's WebOS stood a chance. I still own a Ubuntu Phone that I don't use - single script QML apps would have been the killer, but all that's passed now.
You mean Microsoft? No backwards-compatibility with Windows Mobile to begin with (so companies can't reuse their existing investment into line-of-business apps on actually nice modern devices either), then they reset the ecosystem 2 times (once during the WP7->WP8 transition, another time during the Windows 10 transition).