Aw honey I hope my comment about her didn't upset you. I think she's talented (at marketing).
But anyway, it's a topic which asks a question, which I answered.
Then Beyonce ... (only got one hit last era her album sales are LOW for a icon of her status) one big tour does not make you a touring act. She will have more kids and focus on Business or something. she will always be relevant but musically she has peaked and I only see her going down.
Then Katy (Prism has already shown the start of her decline)
Rihanna will last the longest as her team will be able to market her towards whatever trend is happening in music.
Gaga >> Beyonce >> Katy >> Taylor >> Rihanna
You talk about Beyonce & Katy as if Rihanna's sales haven't gone down . Beyonce has 3 tours over 100M (one she did with 19 dates) also the highest grossing tour by a black female artist ever and total career tour gross is $561M. Beyonce's star has only gotten bigger. Keep waiting on it.
Gaga, she's a mess these days, her last album flopped and she even said she wanted to stop singing and start a family before Cheek to Cheek... she'll be gone they should never have given her so much control in the creative process!
All but Taylor are versatile and creative enough to remain relevant in music, especially Rihanna & Gaga.
Taylor is country/pop and she has a strong fan base, that's her saving grace.
I just don't see Gaga giving up, she seems the most bound-and -determined to remain, successful or not.
I see Taylor getting bored first. And after her, maybe Katy.
I don't know about Rihanna, she could go either way, she's already been in the game so long.
Beyonce definitely is the least likely to leave the scene.
Rihanna is on the way to becoming this generation's Madonna. She'll be scoring #1s in 6 years time for sure.
Beyonce is the closest thing (although in reality still not that close) to this generation's Michael Jackson. She'll obviously be around in 6 years time.
GaGa will become even more of a niche act than she is now. She'll be on her 6th/7th album by 2020 and it won't be of interest to the mainstream.
Katy has a limited shelf life but she'll probably enjoy small success in 6 years time. She's the same kind of popstar that Britney was. The music was nothing special and she appeals to a teenage audience. Britney broke out in 1999 and started to fade worldwide by 2011 (12 years later). Katy broke out in 2008 so 2020 might be when her star fades.
Taylor won't last as a pop act. Pop fans will soon get bored of her and her Country-Pop fans have probably all abandoned her.
. Beyonce has 3 tours over 100M (one she did with 19 dates) also the highest grossing tour by a black female artist ever and total career tour gross is $561M. Beyonce's star has only gotten bigger. Keep waiting on it. 
they should never have given her so much control in the creative process!