The problem isn't the app gap on Windows Phone, sorry big fan here using a 950XL, but it's dead.
The problem is getting developers to write apps for the Windows store at all (For Windows 10), so many companies think, "We have a web page so why do we need an app at all", the company I work for is one of them.
If I search the windows store using our companies name there is one music download, nothing else. Yet they have apps for Android and Mac, nothing for Windows, and it is this missing button on most "Download our app" areas with a Google and Mac button, nothing from Windows of any version, that is the longer term issue.
By abandoning WP MS have almost shot themselves in the foot regards the whole UWP push, if they abandon UWP, then Windows on tablets makes less sense, and Windows is looking at a slow death, then MS becomes a dead giant waiting to be picked over by asset strippers.
I actually believe MS will produce a handheld device, it wont be called a phone and it will run Windows 10, not Windows mobile or Windows Phone, and maybe they can keep the UWP strategy, but they are going about it in the worst possible way, alienate your fans, turn your customers away, give them no hope for the future.
Will the WP app gap ever be sorted, NO it's dead. Will the Windows app gap be sorted, possibly but only if MS get back into the "No it's not a mobile its a small PC with enabled telephony" market and then bloody well stick at it for more than 18 months and sell the thing.