In that case, what happened to mainstream music is overplay of certain songs creating a lack of exposure for different sounds. Its as simple as that.

Yeah, since the release of the iPod and before, but mainly when the iPod came out, it was suddenly "cool" to listen to music all the time, unfortunately, people who weren't already listening to music generally have terrible, terrible music taste, and so they cashed into the glamour/looks based bands that plague music television these days. This led to huge sales of crap music, and thus lead to the problem which you call the "decline" of music. In fact there isn't a decline, it's just that there's so much crap out there that it's hard to find the good stuff. take your time and you will.