Not only that, but developing for Windows Phone 8 required using Windows 8 (which was a flop)
That being said I don't know that there was a path to success. I don't think it was as much about the little apps as the big ones: Instagram, YouTube, Pokemon Go... Google in particular got to play the role of anti-competitive
Word on the street was Google didn't want to bother with WP because they worked so hard on WinCE apps and all of that was thrown away. Probably not the whole reason, but they did make an exchange connector available for gmail on wp, so it's not like they didn't support the platform at all..
YouTube in the browser would have been fine if the browser wasn't terrible.
I definitely felt the lack of Pokemon Go ... but given it released in mid 2016, it's pretty clear why it didn't show up on Windows Phone / Windows Mobile... the writing was already on the wall by then and Windows development would have likely trailed anyway. WP8.1 came out in April 2014; it was an improvement, but you really wanted a follow up release. WM10 came out November 2015 and wasn't very good unless you had the flagship phones, which nobody would buy; I did end up running one of the last builds of WM10 and it actually got pretty good (other than Edge still had terrible UX)... too bad it happened so late. Windows Phone's strength was at the bottom of the market --- cheap phones that were usable, but cheap phones didn't run WM10 well, so the huge global market of people who wanted a smart phone but only had $100 to spend was left for Android.
If you look at sales numbers [1], things were going ok until about q3 2015. A dip in Q3 is kind of expected, because WM10 was coming soon, so people are likely to wait ... but then WM10 is crap and people buy an Android or iPhone and there you go.
[1] https://mspoweruser.com/revealed-1-2-million-windows-phones-...