^ And Justin Bieber concerts do not sell out? Or why do you say then that Bieber has declined but Beyoncé did not?
I say his popularity has declined largely due to his behaviour over the past year. Personally, I used to actually like the kid. He was just a kid living his dream and I do like a small number of his songs. It's catchy pop, fun to listen to sometimes! Then he tried to put on this 'bad boy' act and starting doing stupid shit and becoming increasingly disrespectful at times:
• Spitting off the balcony (The tabloids say onto his fans, but his fans say he was doing it into some bins. I'm not sure which, but regardless it's still disgusting).
• Urinating into a restaurants mop bucket.
• Speeding dangerously through the streets of his gated neighbourhood, despite being repeatedly asked to slow down by neighbours.
• Wrote in a guestbook at the Anne Frank house about how he hoped Anne Frank would've been a Belieber (which many considered disrespectful given the nature of the location he was at).
• Caught smoking, what was apparently, marijuana back in January. (I'm honestly not really against him doing drugs, he's a teen he's going to experiment, but there are a number of parents who wouldn't like it due to the fact he's a role model for their children).
• Cops in Sweden found drugs on his tour bus (While I'm not sure if they were Bieber's drugs, it still reflected negatively on him as it was his party bus).
• Constantly turning up late for his shows during his Believe tour (Keeping in mind a number of his fans are younger and do not have the privilege of staying up late, thus they can't spend all night waiting for him).
I've just noticed a lot of people's attitudes (both in real life and in comments online) have gone from "Oh he's just a kid doing what he loves, he does nothing wrong, leave him alone" to "He's become such a brat, he's so disrespectful - it's like the fame has gone to his head" over the past year or so.
That's why I say his popularity has decreased, because his negative and repeated disrespectful behaviour have caused people to not like him as much. It would certainly turn off some fans as well. Compare that to Beyoncé, who has pretty much always had quite a positive public image and I hope you see the point I'm making. I certainly don't hate the kid (I don't hate any artist for that matter), but I'm personally not as fond of him as I was a year ago.
Returning on-topic however,
I think the song that would have the most chance at reaching #1 (that wouldn't be a duet with a popular living artist) would be Slave to the Rhythm, I think it's one of his best unreleased works! Although I feel like the album itself is more likely to top the charts than a single.