There is still an underground...its just not your underground. For the record, the emo, or scene trend was a failure for the most part. While some bands did well for a year or so, most of the successful acts werent even emo (modern day emo of course...Im not getting into the "true emo is dead" argument). Fall Out Boy, Panic!, and a few others...but bands like Senses Fail, Thrice, and many, many other "next big things" bombed once they tried to move beyond just a large cult following. Bands like Coheed, Avenged, MCR and others arent even emo but were the biggest sellers in the emo trend (proving that no one understands musical genres these days).
The result is that the entire scene trend is collapsing under its own overhyped weight. Mainstream kids stole the poppier bands (like Plain White Ts) and the mainstream culture (as always) co-opted just enough of the look...while shunning the now ridiculous girl pants, massive ear holes, facial piercings and crooked hair cuts. With no support from the underground since the trend went mainstream, and little interest from the mainstream, most scene bands will be dropping off the face of the earth in the next few years. My Chemical Romance COULD remain popular, as they were never emo anyway, and have grown to include influences from the 70s (The Black Parade is essentially a 70's arena rock cd with some Smashing Pumpkins thrown in). Very few others will remain relevant (I believe the world is realizing that Avenged Sevenfold is the worst Power Metal band on earth...and my HOPE is the Coheed and Cambria will get the respect they deserve).
So...to make a short story long...there are new potential trends bubbling up...you just havent heard about them...and the press...like those disrespectful jerks at AP Magazine...are too busy wondering why their cover stars didnt get famous to actually kick the corpse andd start promoting a new trend. As usual, the true trend setters on the street will make the next trend viable, only to be stolen once again by the media and the mainstream.
And the cycle goes on and on.