What makes mainstream music 'dead'? I'd argue the contrary, from my perspective, because now the mainsteam is being permeated with more and more rock/alt/punk type bands whilst still maintaining the archetypal pop, rap and indie which has been dominating the charts for however long. Mainstream music isn't dead, your interest in it might be.
I'd agree with Keith's sentiments though I'd say it's been more prevalent since the start of the 2000s.