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 who was going to buy WM10? And upgrading to WM10, when available, was often a bad experience.
Also, mobile Edge had a nicer renderer than mobile IE, but it was sooooo much worse UX (laggy, slow, navigation buttons went into some sort of button press queue to be resolved seconds later). When you've driven away app developers, ruining the browser isn't a good choice.
So, it's not that you needed more money (although I'm sure it would help), you also need to not abandon the market niche you found in search of an unobtainable, but potentially more lucrative one, and you need to make releases be consistently better each time. (It would also help if one of the big players stumbled, but you can't count on that).
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 Google hadn't gotten underway yet.
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...
Yeah, Microsoft had an exceptional OS that was competitive to iOS (and much better than Android) on its foundations and APIs. It had everything needed to succeed and get a big chunk of the market, except for a clear leadership.
Now, here we are, with extremely powerful Android phones that do little because they don't have a solid software foundation with which to build good software, an iOS has all the high-quality software.
We just retired a Lumia 950 XL in my household. It's an all around better experience than Android in my experience, but there wasn't much point in continuing to push the boulder up the hill. And I liked the ability to pop in a micro SD card, which Apple doesn't support.
From my vantage point, Google did everything it could to kill off Windows phone. There was the big spat over YouTube, where Google wouldn't write a native YouTube client and banned Microsoft's. Google bought SoftCard (I think?) and subsequently killed off NFC payments for Windows phones. When they bought Waze, they ceased all development for Windows mobile, allowing that application to atrophy.
There were certainly a lot of other reasons Windows mobile had difficulty. Not the least of which is developers didn't want to have to manage apps for yet another platform. It looked to me like Microsoft was making good strides there, nonetheless, with some nice tooling. I don't use more than ten apps with any regularity and there were solutions for each of them on Windows mobile, at least.
But, rather than make its apps available everywhere its users were, Google used its market position to starve a competitor. And it wasn't merely a case of deciding not to build apps for it. They took active actions to try to kill off Windows mobile before it had a chance to grow. I see no reason to believe they wouldn't do it with any other new entry. We're just stuck with a duopoly now.
Man, that's a lot of commitment! I gave up when an app I was using for work ended WP8 support and I updated my Lumia 640 to WM10 and wasn't happy with it (Mobile Edge is really terrible, but I already complained), I could give up that app and go back to WP8, but I wasn't willing to live with the notification center bugs in WP8 that were actually fixed in WM10. I still miss live tiles, and the janky photo uploader app I wrote. :(
The Lumia 950 XL shipped with Windows 10 native and was Microsoft's flagship phone, so it got updates for longer. But, yeah, live tiles were great and the UI was just super smooth. The dedicated camera button was fantastic. Being able to access Office apps in Continuum was really neat. I'm sure I have the timetable wrong, but there were features in Windows 10 mobile that took years before they were available on other platforms.
For now I'm using Square Home with a Samsung Galaxy, mostly because I like the hardware options on Android better. But, with the whole industry shifting to locked down devices much like iPhones, I may very well switch back to iPhone for privacy reasons. I really liked having a viable third option. C'est la vie.
But it's still interesting how they initially were late on mobile, with all the FBML embedded things (Zynga games) not working on mobile and things moving from their "platform" to mobile apps, but I assume for now with WhatsApp, Insta and messenger they "own" a notable amount of mobile screen time. Missing the underlying platform of course gives them "neutrality" that they can be on all OSes (till the privacy enforcements make it harder for them) instead of having to differentiate the Facebook phone from iPhone.
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 who was going to buy WM10? And upgrading to WM10, when available, was often a bad experience.
Also, mobile Edge had a nicer renderer than mobile IE, but it was sooooo much worse UX (laggy, slow, navigation buttons went into some sort of button press queue to be resolved seconds later). When you've driven away app developers, ruining the browser isn't a good choice.
So, it's not that you needed more money (although I'm sure it would help), you also need to not abandon the market niche you found in search of an unobtainable, but potentially more lucrative one, and you need to make releases be consistently better each time. (It would also help if one of the big players stumbled, but you can't count on that).