The reasons MS's apps on other platforms seem "more complete" is because those other platforms are far more mature than WM10. It's hard to develop good apps when the underlying foundation is still quicksand (although it appears to finally be hardening now -- well, at least it's not as soft.) Those other platforms have a half-dozen year's head start. MS is in for a long uphill battle, but they are banking/leveraging on their biggest asset, the Windows OS and its vast base of customers, and hoping to pivot that back into phones. They had to merge the two OS's (desktop/OS) for this to happen. If WM10 was a completely different OS, who would develop for it? For such a small existing market? At least by merging the two OS's, they've given themselves a chance that developers will develop for both, with minimal effort in addressing the differences in hardware. They took a big hit in doing this merger. They had to leave a lot of existing phones behind in doing so. And I rather doubt it was MS's intent to leave 50% of their 8.1 phones behind -- they really did try hard to make them work with WM10 -- that's why the official WM10 release was so delayed. I think they decided to just cut their losses without completely killing the platform stone dead.
It remains to be seen whether the gamble they made will pay off.
In the meantime, they are doing what they do best, develop software regardless of platform. That's how they got their start so many years ago. By having a presence on iOS/Android, and by having Azure, they are assured relevance in the future, no matter the fate of W10.
And for all you people who think they should be moving faster in WM by producing more phones -- this to me seems like a dumb strategy. Clearly, the OS isn't quite baked, so releasing even more phones than the 950 would just cost them development/manufacturing money with no clear benefit. Wasn't it bad enough that the 950 was released before the software was finished? Why add even more hardware to this mix? They wouldn't sell that well because people would know/complain that the OS isn't done. Better to retreat and reload, and finish the desktop/mobile OS merger. Which is exactly what they are doing. Is this the right strategy? Time will tell. Was it an ideal way to go? Absolutely not. But their fate on this was sealed many years ago when they stood on their thumbs when the iPhone came out. They are doing the only thing they can do now to try and recover from mistakes in the past.