Re: It wasn't just apps
> I thought the orphan devices had a lot to do with it - i.e. Windows Mobile 6.x devices couldn't run Windows Phone 7.0. OK, as expected. Windows Phone 7.x devices couldn't run Windows Phone 8.0. WTF? And then again for many (but not all) Windows Phone 8.x devices with Windows Phone 10.0!
It wasn't just the devices that were deadended, it was the apps and the whole development toolchain that was dumped and reset. WM6.x was a complete dead end, nothing could be ported to WP7*. At least there was a conversion that could be done to some WP7 apps so they could be loaded into WP8 - when they didn't fail due to incompatibilities. WM10 then moved to UWP but developers had lost interest in rewriting everything yet again.
* WM6.x devices outsold WP7 for many months after WP7 release due to businesses needing new phones that ran their existing in-house developed apps that couldn't be ported to anything else and needed completely starting from scratch again.