Ok, I'm not thingking about Aaron Carter or the Spice Girls...
You can always find negative examples, but where are the positive examples of great mainstream artists today?
I think that some of the most successful artists of earlier decades are among the best artists of that time. That isn't the case anymore.
Not sure if those are the best examples, but I'm thinking about The Beatles, Rolling Stones, Elvis, Led Zeppelin, Pink Floyd, Eagles, Eric Clapton, Dire Straits, Queen, Van Halen, Metallica, Iron Maiden, Run-D.M.C. or even Michael Jackson until 'Bad'. These were all mainstream successes.
Some of those band still release new albums, but they're mostly crap.
Okay, yes, there may be more examples from the past that we can point to as mainstream artists who were actually good, but that's kind of the hindsight is 20/20 argument. In thirty years there will be current mainstream artists that will be praised and held up as examples of how great music was in the 200x's / teens, but those artists will be greatly in the minority, as they always have been, to the total number of recording artist.
Since others are listing some of their favs I'll do the same
The New Pornographers / AC Newman
The Flaming Lips
Animal Collective (some)