Earlier quoted context omitted.
Microsoft actually had a winning formula. Make cheap phones that work alright. And they were getting market share with later wp7 and throughout wp8. Between wp8 and WM10, they merged the phone team with the regular OS team, and eliminated (or at least gutted) their QA teams, and they decided to target the high end market only . There were no low spec WM10 phones, and there hadn't been many high end buyers anyway, so…
IIRC Microsoft didn't eliminate their QA teams until after Windows Phone 10 was mostly dead. Regarding apps, bear in mind that Google went out of their way to prevent Youtube and their other services from working on Windows Phone, e.g. https://www.theguardian.com/technology/appsblog/2013/aug/15/... . There's nothing that Microsoft could've done to overcome that much of a disadvantage and anti-competitive scrutiny of…
Also, I had heard Microsoft blocked 3rd party browsers early on, when Mozilla wanted to develop Firefox for WP, that would have been a lot nicer than mobile IE and later Edge, and probably would have run YouTube just fine.
On the dates, I see a RTM date of Nov 2015, and GA of Mar 2016 for WM10; I think the end of QA and the merger of Windows Phone with Windows was actually at the same time, Nov 2014, AFAIK. It's a bit tricky to nail it down, but I think this article, read with hindsight speaks to those changes. https://www.zdnet.com/article/microsoft-layoffs-operating-sy...