The thing is, there was some decent support with WP7 and WP8. Looking back, with all the reboots, I think that killed developer support, and it never regained its position.
The reboot to W10M in retrospect is what killed the platform (even though it was already coughing up blood, so to speak); flagship priced hardware, with not that flagship feel, really buggy OS, which was a complete turnaround from WP8.1, I think windows 8 on desktop also poisoned the well somewhat, and lastly, developer support just dropped off.
Going on the app store, some of the better games on there are still WP7 apps. While WP8/8.1 was undoubtedly the best phone os, it was completely pointless in terms of being made obsolete.
The relaunches were a mistake in my opinion, had they just tacked on new features to the OS, and made it relatively seamless for the users and developers, rather than whole new OS, then it would have been much better.