Personally I think the year between announcement and delivery of a the Windows Phone killed them. A year is a long time in tech and the game was lost. Everyone was excited when they first showed the demo. But you kind of have to use that excitement by delivering something pretty fast.
Apple used to be the company that announced things and then said you could buy it the following week or so. When Apple started these long lead times (HomePod, Airpod etc..) thats when it all fails. Just have it ready.
Also, MS could have used Silverlight / WP or whatever for a tablet to battle the ipad but they wanted Windows OS to be dominate everywhere. So they took ages with that and the same result. Time is everything in tech.
In 2015, a few months after I finally threw in the towel on Windows Phone realizing that it wasn't going to survive much longer and that Windows 10 was utter crap, I looked back and in hindsight will concur that they were always about a 1-1.5 years behind from the start and that's probably the single biggest factor in Windows Phone's ultimate demise.
Even the limited app selection was largely overcome by 2015 through third-party apps and Microsoft's in-house efforts to bridge the gaps. The whole time, Windows Phone users kept holding out hope that the next version, the next update, the next phone hardware, or that one missing app would change everything. But it never did, and the goal posts moved every time, whether it was WP7.5, WP8, WP8.1, cheap phones, flagship phones, Nokia becoming an OEM, Nokia being purchased, Instagram, Snapchat, or whatever other end goals we thought would finally convince mainstream consumers that Windows Phone was at least a viable option over iPhone or Android.
Meanwhile, it got harder and harder for core enthusiasts as they saw other innovation happening on iOS and Android that Microsoft just took way too long to bring over. Notification pane and copy/paste were basic ones, but we never got a real mobile wallet (other than that janky Amex prepaid card thing that barely worked), voice over LTE, HD voice codec support, or Wi-Fi calling; even on AT&T, who was labeled the "premier" carrier for Windows Phone in the US. I know AMR-WB and VoLTE code was in some of the late WP8.1 releases, but Microsoft never worked with any carrier that I'd seen to get it functional. To this day, if I put a SIM in a WP8.1 phone VoLTE nor HD voice work.
On top of that, Microsoft shot themselves in the foot a couple times by letting Nokia release flagship WP7 handsets, then drop WP8 a few months later with no upgrade path. Then they kept rebooting the SDKs (sort of the same way Apple has been with watchOS), so every time an ISV would spend months or years building their Windows Phone app; Microsoft uprooted everything and basically made it so that if you wanted to take advantage of any of the new hardware functionality or features of the newer OS versions (better multitasking, background tasks, etc.) you had to almost rewrite/rebuild from scratch. And they did that 3 times in 5 years!
I don't blame ISVs for giving up and just pulling their apps. Otherwise you end up with a support nightmare of rabid WP fans constantly emailing you and giving bad app reviews because your app is slow and doesn't support "X" feature that's been out for over a year, or you dump a fortune into app dev for a platform with <10% market share. What scares me is that I think it foreshadowed what's happening on Apple Watch, where eBay, Target, Twitter, and a host of other major apps dropped their Watch support over the past 2 years. Apple Watch isn't ubiquitous
enough for dedicating development to the platform at a loss and Apple keeps upending the Watch SDK so that old apps that were slow, reliant on the phone connection, etc. had to be updated or dropped from the App Store. That reliance on the phone should've never been there from the start.
Those three years they took from the iPhone launch in 2007 through the Windows Phone announcement in February 2010 was just too long. If they'd been able to get WP7 out the door in late 2008 or 2009, I think they could've been where Android is today or at least split that portion of the market. Or if they had pulled an Apple and came out of the gate announcing Windows Phone 7 with devices available within a week or two...that would've changed everything. They lost too much momentum in 2010 and competitors had time to start preparing and catching up.