Carly
Dev
BOB
Some artists peaked within the first 2 eras but still here because of somewhat fanbase and quick little singles.But forever will be associated with 1999-2002 or 2008-2010 (for example).
Lady Gaga (shorter than what everyone expected it to be)
Nelly Furtado
Ashanti
Brandy
Iggy (I think her career is coming to a close soon)
Carly Ray Jepsen
Kesha (she declined really fast)
Paula Abdul