I think another reason why rap is so popular right now is because you can dance to it. Rock 'n' roll was originally music that was made for dancing, but as it got more intellectual and then heavier you heard it less when you went out for a night on the town.
Arent you forgetting the new crop of bands like The Killers, Franz Ferdinand, etc? IMO, you can dance to their music.
Whereas the Crue combined the badboy image with the danceable tunes with the great guitar sound, none of the new bands I've heard incorporate all of these RNR characteristics. It's like each of these bands is missing a piece of the puzzle.
Rap hasn't replaced rock at all
That is the entire premise of this thread... the original poster asserted that rap has usurped many of the characteristics that were once associated with rock, and hence became more popular. Judging by the replies, most seem to agree with that.