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Google had apps for Windows Mobile 6, and Microsoft chose to throw that app ecosystem away for Windows Phone 7. WP8 could run WP7 apps, but for the best experience, you needed to redo a bunch of stuff in a way that only worked for WP8. Same thing for WM10, except that this time, Microsoft called it Universal, although a Universal app didn't work on the WP8 and WP7 devices; and AFAIK, WM10 never surpassed 7 or 8 in active devices; whoops.

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...