Coming from Symbian, when I first transitioned to WP8 on my Lumia 920, there was certainly a lot of shortcomings on WP then. There was no USB OTG which Symbian has, there was not file explorer, and the list goes on. But somehow I got used to it and come up with other turnarounds. We humans adapt fast. And four years since then I get to see some features coming back in W10M. From here, it seems to me that WP/W10M development in basic functionality is rather slow. Sure it may other niche features but basic features that user like me used to have on Symbian came back rather slow. And during this period, Android and iOS had started to catch up by giving basic features that many users are crying for. But MS has been rather slow. That could be why users left after trying WP and not many from other platform wants to try out WP after hearing so many shortcomings from others and also from some biased tech review websites.
This could have made the user growth slow or stagnant. With that, apps developers are reluctant to build apps for WP8. And with not many apps, many more users jumped ship. It's the whole chicken and egg cycle. People do not want to use WP because of the app gap. Developers do not want to build apps because there is not enough users. Then come W10. Universal app platform is a good idea. Creating more user base because apps can be used on W10 and W10M. But does that mean developers have to rewrite the app? If user base has been low for all this while, it's hard to get them to re-write, I guess. Yes, MS can promise the whole W10 will have a lot of users thing but, are developers keen? Project Islandwood is a good idea but what would be the motivation factor for an iOS app developer to create app for W10M. Yes, it might be easier because of Islandwood but what would they get other than that? How is MS attracting them?
As a user myself, I will promote W10 to families and friends but how much can a small fry like me do to bring up the market share? And with a majority if tech websites labeling WP/W10M a dying platform.... Are they just being biased, or are they not familiar enough with the whole WP/W10M as they themselves haven't used WP? Or are they speaking the truth?
I like WP/W10M. Having a secondary Android phone (Asus), WP is cleaner than Android. Widgets could not compare to live tiles. Also, when using Android, I always have this fear that there could be spyware, viruses or hackers targeting my credit cards, banking information when I am performing online transactions, yes even through banking apps. But I feel secure when WP web version. It could just be me being paranoid but I feel that Android and iOS are major targets for the scammers and hackers because of their large user base.
The thing I love the most about my Lumias are the camera. I don't have fancy a DSLR and my current 950XL is my primary camera for taking pictures and videos of my kids. Actually my Nokia N8, 808, Lumi 920, 1520 used to be my trusted primary cameras. They created wonderful memories for me.
Okay. Enough of the long writings. I like WP/W10M. There are app gaps that I wished to be filled but by saying "I can live with it" it's me adapting or bending myself to use WP/W10M because of the clean, somewhat secure platform and to be able to get the best camera. So it all depends on our priorities I guess.

. If those missing apps are really important/cruial/mission-critical to you for personal or work reasons, then it might be wiser to switch to Android or iOS.
Haha, i really miss a local shopping app that I have on my Android but not on W10M. Thru the app I can get discounts but using the web version on W10M, I don't get discount :-(