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! After all, for corporate users missing Snapchat and Youtube apps isn't a problem, but devices with a support lifetime of 18 months was.
When you jerk your customers around like that, it is really hard to build any loyalty. Especially since they'd seen Microsoft able to support Windows upgrades on a near infinite combination of 3rd party hardware but somehow couldn't manage that feat with phone hardware they made themselves!
Had they introduced Continuum in the 7.x days, even though it would be pretty slow, just the view people would have of being able to run their Windows PC applications on their phone in a few years time might have got a lot more people interested in the platform. Maybe not the consumers, but definitely the corporate types.