The current thread on Lindsey Lohan brings the thought to mind. As someone pointed out there, she is swiftly approaching Hiltonian levels of being famous for being famous. Once gravity, time, and cosmetic surgery have their way with her, she seems likely to fade out of sight.
On the other hand, I can see Alyson Hannigan working steadily for as long as she feels like it. Comely though she may be, her appeal isn’t based entirely on her looks; in fact, I think her personas add to people’s perception of how pretty she is, and simply like her as an actress.
Anybody else?