The thirst for pop girls is so real on this forum.
Selena Gomez, Demi, and Ariana are all never going to be major Top Tier popstars. Their fan base is primarily made up of the tweens and teens who stream music and don't buy it. Their music is manufactured and created for them and none of the girls have an artistic identity. Their sales are never going to be impressive and no adult is going to take them seriously. Just be happy that Selena is having a successful era on a personal level.
The only girl who is on her way to the Top Tier level is Miley Cyrus. It's always been this way, even during the Disney days. Miley had the best and most successful show, she was the most famous, and literally outsold these girls's discographies with the Hannah Montana soundtracks.