Never. They have existed since the good ole\' days. Remember The Temptations? Four Tops? The Pengiuns? All boy bands. They would sing at the time popular songs and dance. Agree\'d, the first two I mentioned was Motown "soul" artist, but in the basic idea and form they was boy bands.
Boy Bands did go down abit durin\' the "grunge" scene. But they was still there. After NKOTB (new kids on the block) went away. They was still here. Just not as popular. There was bands like TAKE THAT, All 4 One and such.
And with good reason. Most women love boy bands. You gather abunch of "cute" guys singing love songs in perfect harmony and women will go crazy for it. The true success for a boy band is when they can attract the pre-teen, teen, adult crowd. N\'Sync has done exactly that. And, unless by some chance the next album is a complete waste of airplay (i.e, no radio hits) then they will continue to ride the wave of success.