What metric are you going by then? Because an artist's perceptual appeal is something you can't really quantify, but sales you can. If we're going by "shock factor" alone, of course she's going to reach a plateau where no one's really surprised, but the key is that people are still checking on her.
Messiah is looking FAB.
I'm not going by a metric that exists yet. ArtPOP is not out yet. When it comes out and if it slays I'll be a believer in her longevity because by that point nobody will be able to make the case that she's still riding high off the hype generated from her debut.