There are people who don't have this determination, time, money, craze, or whatever, and what to learn self defence (and along whatever comes with it) in just one system.
Those people probably should not be studying a martial art at all. They should find a set of techniques and strategies (almost certainly not all from "just one system") that will enhance their safety from whatever it is that threatens them. But until you can say what that threat is, you're in no position to be either proposing or dismissing any solution. It's like walking into a hardware store and saying, "I want the best tool in the store!"