IIRC his latest album, Journals, absolutely tanked commercially (there's really no hit singles from it as opposed to the 2 or 3 massive songs each he got from Believe and My World 2.0). While the Biebs still has the potential to rebound and make another chart-topper, he's no longer the absolute unstoppable force on the radio that he was about 1-2 years ago. It looked like Journals was an attempt to rework his sound, and I've heard a lot of good things about it, but the radio just didn't buy in.
Next couple years are gonna be crucial to Bieber. He's got a lot of odds going against him in the coming few years, such as:
- the growing backlash against him for idiotic acts such as pissing in a bucket at a restaurant, beating up a taxi driver, just acting entitled, etc
- the big turnover in the music industry that's been happening lately - the generic club beats that Bieber usually sings over are getting phased out by 70s throwback jams (I'm talking to you, Pharrell) and more experimental (at least in terms of pop music) beats. This year alone the songs "Talk Dirty" and "Problem" are both chart-toppers and both of them feature a good sax line that up until now was really uncommon in pop music. Biebs will have to adjust to these new beats and find a way to work them around his style.
- father time. it's simple, it's easier to sell your music to young kids when you yourself are a young kid. Bieber's just turned 20, but that's pretty damn old for a disney-type star. Bieber will need to adapt to selling his music to an older demographic soon, and it's hard when most of the said older demographic already hates your music for originally pandering to the preteen/teen crowd.
It's not impossible for Bieber to overcome these struggles, but not every child superstar can be a Justin Timberlake. Time will tell, I guess, how long he can stay relevant.