#1 Ciara - Ciara has had 2 back to back commercial failures before the age of 30. Artists that generally rely on dancing usually do not see major careers beyond their 20's unless they made a huge mark or left an imprint in their 20's. These artists also rely heavily on youth and sex appeal. Turning 27 this year, and her first single failing under LA Reid, it doesn't seem likely she will see success again.
#2 Katy Perry - The moment music shifts is when Katy will be done. Unlike her similar pop counterpart, Rihanna, she won't be able to keep up with the transition out of bubbly pop and dance music. With no Grammys, and not a lot to show for outside of Billboard, it doesn't seem she is making a big enough impact outside of airplay and iTunes.
#3 Jennifer Lopez - We are seeing the last days of J. Lo the artist. She's over 40, her albums don't do well, her singles don't really do well, and without American Idol, public interest is about to dip back to where it was before she was on TV every week. Hopefully, she can get her movie career back on track. But Jenny's about to be back on the block.
#4 Nicole Scherzinger - I'll never understand how someone can make such great music as a lead singer (and the only singer) in a group, but makes absolutely horrific music as a solo artist. She looks like the complete package, she dances like the complete package, she sings good enough to be a complete package, but her music and personality aren't strong enough. And well... she's not likable.
#5 Christina Aguilera - Don't let "Moves Like Jagger" fool you. Christina will not be returning to the days of album sales. The single might do okay, but the base that grew up with and loved Christina is far gone. She, if lucky, will score a digital hit with Pop radio support. But that's about....IT.
at ya'll saying Katy, Kesha, and Gaga. Those three are going to be the top pop stars for a long time.