Reality:

Madonna, Mariah Carey, Whitney Houston
Secured a seat in POP Reign long time ago.

Just Recently,
Britney Spears, Christina Aguilera, Beyonce
also secured their seats...

at this era
pop divas who will more likely to secure a seat are:
Lady gaga, Rihanna, Taylor Swift and Katy Perry (All already proved that they could have number 1's, win awards and its about time for them to prove longetivity like the britney,xtina and B)

Other on the list of new artist could also secure a seat.

About Adele no one can tell what is she going to be by the coming years but I like her and happy for her. I hope she could also secure a seat for herself.