For the same reasons people vouched for Necromancers, Tinkers, Spellbreakers, Monks, Demon Hunters, and Death Knights.
Some of these classes have been implemented and not in the way people ever expected. I'm not sure where you got the idea that Dark Rangers could only happen through those two methods, but the classes are all possible. It's just a matter of how/when. Blizzard has shown they are willing to rework an existing class to establish a new one. They are also willing to surprise us with the introduction.
If we get a new class, I expect it to be cosmic themed to some capacity due to the story direction.