Yeah, the idea that Nadella killed Windows Phone only makes sense in the context of Windows Phone already having failed under Ballmer.
I was a Windows Phone user during 8 and 8.1. There was a short period where I felt like some traction was taking place. My bank even had a Windows Phone app, until they didn’t.
Windows 8.1 was the most competitive version against contemporaries, but then the delays and issues surrounding 10 really took the wind out of those sails.
My perspective as a consumer is that Microsoft buying Nokia seemed to have made Nokia worse and delayed their phone development process. I found myself without any upgrade path, while Apple and Samsung users could get a pretty significant upgrade in capability every year at that time.
Nokia was also better at making low-end phones and had very few flagship products that were iPhone and Galaxy competitors.
On the business side Microsoft didn’t focus on having their entire lineup available on all four US carriers. They had all these weird carrier exclusives where getting a new Windows Phone would mean switching carriers.
I have to think that the break in compatibility between Windows 7 and 8 really screwed over developer relations as well. On the Apple side they were delivering an experience very familiar to Mac developers, and on the Android side the experience was an open source free-for-all playground.