When people make the choice to buy something they discriminate, according to their choice and needs. That's market forces and simply put people buy what they want. And that isn't a BlackBerry or indeed wp - hence the sales figures. The roi isn't there.
People do not even have full choices on phone OSes when they do not have app neutrality.
There are many people who like Windows Phone and BlackBerry OSes but they cannot use either since there is no app for these two platforms. Without app neutrality, people do not get the full freedom to choose what they want to buy because of the app restrictions.
Think about how many times you have heard "I like Windows Phone OS, but I cannot buy it because I cannot get ______ app on it".
People are buying phones based on app availability and not based on the actual Phone OS it self. This is resulting in the duopoly of phone OSes leading to the staling of phone OS development as you can see with IOS and Android phones recently.
Also, app neutrality does not necessarily mean every single private app developer would need to write apps for all 4 major platforms, so that the small developers go bankrupt.
1) There can be way to protect small developers, so that only the app developers/company that make above the set minimal profit $$ would have to develop for all major platforms. If the developer makes profit under that amount, they would not have to be take part in app neutrality. This would make the developers with big enough profits (Google, Netflix, Amazon, etc) to develop for Windows Phone and BlackBerry, while protecting small developers of each platform.
2) There can also be mobile OS requirement to qualify for app neutrality such as set minimal number of users, and the minimal number of countries it is available in, so that developers would not have to develop for discontinued or all the small obscure mobile OSes.