^ Stephen Elop made the decision of choosing Windows mobile for Nokia. Now works in Microsoft..
You forgot to mention that he worked at MS BEFORE going to Nokia as well.
The phone market evolved, and Nokia didn't. They tried though, more than once, and I'd say WP was one of their best bets. Unfortunately, WP was not really good enough at that point. (The lack of apps was also a problem.)
Later on, WP 8.1 was groundbreaking (by MS measures) when it came, with all these small changes that made it a more complete OS.
Actually, I really like 8.1 so much that I actually would have preferred that MS kept on developing it for another year or so, updating the looks of the start screen and add support for UWP (the latter would probably be way too much to ask for).
Don't get me wrong, W10M is great, but it was kind of half baked when it showed up. Imagine the launch only four months later and the reviews would have looked different.
My point being, it was painful to see MS pretty much throwing out 8.1 a bit too early when 10 wasn't good enough yet, because it only gave W10M a lower reputation that it deserved.
This was supposed to be THE launch of W10M, combined with new high end devices, and the future reputation of Windows Mobile was at stake, so it should have been well thought out. It's not comparable with, say, Samsung launching a product, that's relatively quickly forgotten when there are lots of alternatives and the OS is well known. One bad Android model doesn't make a dent in the Android market share.
Then we have the WP fans waiting for new devices, I guess MS didn't want to loose customers so they couldn't wait.