Here's one data point for you. Windows Store used to have an official Kindle app. It doesn't since the end of last year, because Amazon basically said they don't see the return on that investment. They now recommend their desktop Win32 app if you want to read Kindle books on Windows.
Needless to say, iOS and Android do have well-supported Kindle apps.
(As a kindle user, I too am extremely disappointed Amazon developers haven't yet build a modern kindle reader for Windows 10 and cling to their Windows 7-targeted Win32 apps for now. The Silverlight-based app they built for Windows 8.1 I still use sometimes, and it is woefully out of date with the features of most of their other apps.)
And it is next to impossible to get a refund.
Last week I noticed that my purchased music on Groove has disappeared ahead of the end of year termination of service.
I can't dispute the unpopularity of Windows Phone, but from what I hear the Windows Store is fairly successful in Windows 10. Many consumers use it to install apps, which is a judge of popularity. However, if by popularity you instead mean sentiment, then I get the impression that currently most people are ambivalent about the Store in so far as it is a pragmatic tool that people neither love nor hate, just as most people neither love nor hate their toaster so long as it toasts. (Certainly there are haters, but volume of their voices is not necessarily an indication of their size/number/consumer spending activity per the first definition of a popularity, just a reminder of the passion with which they feel their sentiment.)