Verizon refusing to sell and refusing to allow on their network the Lumia 950 because of a hissy fit that they didn't like how the previous flagship performed and that Microsoft got an almost favorable AT&T deal for the 950 was dumb on several levels.

AT&T getting bored with their 950 deal and then refusing to advertise/market/sell the phone, was certainly a death nail, partly because it was so much easier and cheaper to just micro-manage the Android platform.

It might not have killed the platform in the US if there were more than one phone manufacturer in Windows Phone at that point in time.

(That calls into question if the Nokia buy out was the right move. Which with Nokia last one standing already, it was probably the only move, but the platform had enough market share before Nokia was at risk of tanking that had a couple Android manufacturers gotten fed up with Google at the time things could have gone differently. Though admittedly, armchair quarterbacking is easier with hindsight.)

The app situation was always something that could have been addressed if people were (capable of) buying the platform. The lack of OEM manufacturers and the lack of support/interest from the carriers certainly mattered more than apps at crashing marketshare of the platform below the critical threshold for active application development.