Why are Miley and the other Disney stars fading so fast?

Miley Cyrus already looks to be fading from the limelight as the tweens are now attracted to the more artists like Ke$ha and Katy Perry. Miley's share of the tween market is also being eaten away by newer, prettier Disney "princesses" like Selena Gomez and Demi Lovato (and they too are likely to wane very fast!). The Jonas Brothers have been virtually forgotten and made irrelevant by the likes of Justin Bieber. And who even remembers Zac Efron, Vanessa Hudgens and High School Musical now. The Disney stars who emerged in the past 5 years seem to have a very short shelf life!