I really wanted Windows Phone to do better, so I may be somewhat biased, but WP 8.1 was reasonable, and they were making progress with market share at the low end.
In my opinion Windows Mobile 10 fizzled out because of a few reasons; the most important reason was organizational changes meant nobody cared about Windows 10 on the phone, and the releases were pretty bad for the first year. The other important one was that for quite some time, the 950 was the only phone it was reasonable to buy that had it, and it was too expensive. The 650 was basically the same hardware as the 640, but shipped with WM 10, and was more expensive.
Also, they had promised all WP8 devices would be upgradable to Windows 10, but then they pulled it back for low memory devices (which makes sense, because it was sloowwwwww, but still)
