Notice u said Beyonce....then listed Selena Gomez and Justin Bieber next to her. Firstly, modern day entertainers do not experience as much longevity as elites like MJ, Prince, Madonna etc did. Not necessarily because of quality, but changes in the industry occur quick, so more adaptation is required, compared to back in the day where u could build an identity. Legendary artists could control the industry for about 3 good decades.
Today's relatively big/elite entertainers go for 2 decade as a stretch. Based on quality and longevity the top artists would be Beyonce, Usher, Justin Timberlake, Britney Spears, Shakira...leading onto Lady Gaga, Bruno Mars etc. These are all excellent vocalists on stage (britney spears is debatable), and great dancers. The first 4/5 are commercially the biggest impactors in the modern era, and those i followed them with are on the same track. These pop artists are actually talented, musically dynamic and have a good level of co-writing input into their songs. As for pure singers, Alicia Keys, John Legend, Christina Aguliera, Jennifer Hudson, Adele, Sam Smith followed by Ariana Grande, Demi Lovato etc are the post Whitney Houstan, Stevie Wonder equivalents (not in talent but in their role) all can dominate a stage vocally too.
Now i'll list the mediocrity u were actually referring to that would involve Rihanna, Chris Brown, Justin Bieber, Katy Perry, Jason Derulo, Selena Gomez etc are borderline average, suckling the ignorant modern may music audience with catchy synthetic hits.
All in all, we have talented artists, so i'm fairly satisfied. Nowhere near as good as those who left a big legacy, but they're all still good. I feel like the perception of modern day music is no-talent, lets try and distinguish next time. Putting Beyonce Knowles alongside Selena Gomez in a sentence is unacceptable lol