Of course, Windows Phone suffered from a vicious cycle due to entering the mobile market late. The low market share resulted in significant disadvantages (especially in the key area: third party apps), which consequently lead to consumers not being interested in the platform. This lead to developers not gaining any interest in writing apps.
But its to simple to blame everything on Microsoft's very late move in this critical industry. It's also how they entered the market and how they evolved over the years. The platform had some quite interesting characteristics, such as a digital first UI (which everyone copied / refined over the years), some great features out of the box (like the social media integration, instant translate, and music detection), and covered a wide range of device classes (from low-cost to high-end). However, Microsoft unfortunately understands how to ruin a platform. Updates have been released rarely (any mostly with no real killer features; if they have been good features it took way too long from announcement to deployment) and the changes to the platform have been devastating.
One should make sure that the product becomes actually gradually better - not worse. Microsoft made some moves that went into the right direction, yes, but they also made things worse from time to time. Let's consider Windows Phone 10. The mail client is certainly better, but why can't I set the update interval? Also the calender is basically a joke. It displays a modal dialog on a phone ("saved"). Changing the time feels weird. This has all been solved much better before. Core parts of the OS, such as keyboard gestures (swiping) or Cortana are worse than before. How can that be?
I doubt that the engineers are responsible for this. I know that Microsoft has some of the greatest and brightest software engineers in the industry. I blame the managers. No one who would have interacted with the software for more than 20 minutes would have shipped it (or left it in that state). It feels like Microsoft wants to kill the Windows Phone brand. I really dislike this attitude. That Microsoft also fails to admit that the platform sucks in the current state is symptomatic.
Why all these changes? Windows Mobile to Windows Phone 7 - okay, an ambitious novel way and risky approach. Kudos! Windows Phone 7 to 8. The most unnecessary kernel change ever. There was no shared store, no UWP apps, nothing. If there was some momentum with WP 7 Microsoft certainly killed it here. WP 8 to WP 10 feels like a transition that is both, necessary and unnecessary. I still can't believe how much worse the platform has become in so many areas. On the hand, I see that some improvements are also there. It just won't be enough to keep Windows Phone alive.