The problem is actually not the lack of apps anymore, IMO. WP81 has most of the apps I need, just not the fancy new hyped ones.
The real problem is what made Windows great back then: Support of devices. I'd call this the Device Gap.
Back then, Windows was great because you can plug in anything to your PC, and it would just plug and play - just works. It supports all types of printers, monitors, USB gadgets, high end graphics cards, biometric sensors and anything you throw at it. Mac OSX is still nothing compared to Windows in this aspect.
And then I look at Windows phones, and the devices I have around the house. There had been lots of new home automation and wearable devices in the past decade, and Windows - both phone and desktop - support very selected few of them. I have Insteon, Roku, Logitech Harmony, Philips Hue, Wink devices, Amazon Echo, Chromecast, Xbox, FitBit scale, MS Band, Pebble watch, a handful of Trackr devices, some toy drones, musical instruments that only work with iOS, etc (yes, my router is overloaded). While there are apps for half of them, they are all way behind the iOS / Android counterparts in terms of integration.
I ended up buying a cheap Android tablet that acts as a makeshift 'home automation hub', just so I can connect and configure them. Not having consistent Windows support for these devices limits what I can do with them, e.g. IFTTT automation, Tasker, etc. For example, I cannot have my geolocation of the phone inform my lights to turn on, or change the volume of my receiver since Harmony doesn't have a Windows app. And forget about buying any smartwatches - there is only one choice, the MS Band. It's the little things that grind my interest down little by little...
Having tried out the new Windows 10 Mobile on Lumia 950XL extensively on Device Nights, Microsoft Store and on the Oct 4 #Windows10Devices launch event, I am seriously concerned about the future of this OS. The UI is a few steps back, and the Hamburger menus are truly aggravating to use. I hope they listen to the feedback and fix them soon, like what they had done to the Photos app on the mobile recently.