I won't go in on Gaga like I did with Riri in that other thread because I'm also a fan and what she's accomplished in so little time is no joke.
Time will tell I guess. I mean, Gaga has had extremely successful eras and a few slumps but anyone can have those and recover. Katy in the other hand has been consistent throughout her entire career and has showed no signs of slowing down but no one has it safe nowadays, especially these pop girls because the GP and even the fans in general could turn their back on them once a new IT girl pops up in the music scene.