I think Bey, Rih and Taylor have the stay factor.
The world is simply obsessed with Beyoncé, Rihanna has the versatility and likeability to stay on radio and Taylor has one of the biggest most loyal fanbases.
As for Gaga, who knows, but I think she will always have a place in pop music whether it be as a touring artist or simply a throwback jam.
I honestly see Katy ending up as the Paula Abdul of the bunch.