There are 195 countries in the world. If we assume that each country would want its own backdoor that means there will be 195 backdoors or master keys for every encrypted application.

If we also assume that on average only 100 people per country have access to the back door (a ludicrously low number) that means that around the world encryption relies on almost 20,000 people being completely honest, not willing to be bribed and not capable of being coerced. The likely reality is that the number of people with access to the back door would be a lot higher.

Basically that means no security whatsoever...apart from those really bad people who create their own, after all it is just math.