I don't disagree at all that popular music isn't very good right now. I'm sure a lot of people like it, thus it being popular, but certainly not my cup of tea.. and not yours either.
However, I think that has more to do with just the natural cycle of music. It seems the general style of what's popular cycles every so often (I'll say 10 years for arguments sake). Nirvana and the "grunge" era was 20 years ago. Was Nirvana responsible for the late 90's being dominated by rap? Probably not. People just moved on from rock. Before them it was hair metal (good riddance to it being dead, by the way), before that disco or whatever they listened to in the 70's and then 80's.
Another theory - Do you not remember New Kids on the Block or Milli Vanilli? Shitty music was popular before Nirvana and will continue to be popular. It's not Nirvanas fault that a large portion of music consumers are completely unwilling to differentiate between talented artists and complete hacks propped up by autotune and slick production which offers no substance. Most people who buy and listen to music, don't actually care about music as much as you do. As Kosmo said, good music is out there, it's just not popular, with few exceptions, it's never really been popular. The early 90's, looking back, was a very rare period where good music was also popular, but there was also a fair share of crap at that time as well.
I like Nirvana a lot and think they were highly influential.. but to say that they started a chain of events that led to the soulless vacant music of today, while also causing people on a mass scale to actually like this music is a very large leap. I'm not sure anyone is that influential. It just happened.
