You get these fads where an artist or group becomes hugely successful. Bay City Rollers, anyone? The question becomes one of longevity.
You can argue that the likes of The Beatles, Stones, Elton John, Springsteen, Sinatra, Elvis etc remain popular for years or even years after they disbanded/died.
I guess, the test for the Swifts and Sheerans of this world is, will they still be popular in 10 years time or will they just be the odd track in a playlist? Swift's fan base is huge and she is not dim, so I suspect they'll go with her as she matures.