If you count out pop which is ALWAYS going to be around, as far as I can tell these days 'alternative' IS what I think of when I think mainstream (thus making the 'alternative' label kind of a joke.) What I think of when I hear the term 'rock' all but died out in the early 90s IMO. Now 'alternative' has kind of usurped that mainstream position which is kind of sad in a way. I like a lot of bands like Weezer, Dave Matthews, etc, but my overall tastes generally lean back toward classic rock overall.