Once again, I have to notice how eerily parallel many of the comments here are to what is happening on CrackBerry, with respect to BB10. We love the OS but despair over the apps. In the case of BB10, however, it's now pretty clear that BlackBerry itself has given up, so if there's a future for BlackBerry handsets, that future is Android.
Which leaves W10M as the only currently (semi-)viable alternative to Android and iOS. As an owner and still sometime user of a BlackBerry Passport, I think the app situation for my humble Lumia 640 is in some ways a bit better. Yes, my Passport can run many Android apps, but it's stuck forever at JellyBean, and Android apps are moving on. Kindle, which is one of my most-used Android apps, now takes forever to load, and freezes once in a while after it does load. Tapatalk, which I also use a lot, is barely functional.
In contrast, mTalk on W10M works pretty well. Kindle for W10M is...well, disappointing. As an academic, I mark up my books a lot, so I can find important passages later. I can't do this on the W10M app, and that's kind of a Big Deal for me. I use Pocket quite a lot on Android, but Poki is pretty decent too.
But occasionally when I'm out it'll occur to me to record a television program. I can't do that on W10M, whereas on Android I can use the Verizon fios app. It's a convenience that I don't use very often, but nice to have. And that's how it is with apps. We get used to not having them, especially if we've never had them, but once we get used to having them it's hard to go back. This is the problem W10M faces: If it feels like a step backward, there simply isn't any convincing reason to switch. Live tiles won't cut it.
The only way forward is for UWP apps to get enough traction with developers that W10M enjoys a coattail effect. Developers have to see that the number of installs is going up, not down. That's the only way they'll take an interest.
I think we need to remember that the bigger, more popular apps tend to be the more complex apps, and these cost money to update and maintain. That's why Waze is slow to update on W10, until they see more demand. Same for Kindle. In my uninformed opinion, it can still happen, but it's far from a sure thing.