Brittany never broke out of her “naughty teen sex-kitten” mold. One can probably monetize that image for about 10 years, maximum.
Spears never re-defined herself, and her bouts with addiction really pushed her over a cliff.
Taylor Swift is on the edge of that now. I don’t think she has the addiction issues, and possesses much more music-writing talent, but she also needs to re-invent herself very soon.