Youtube hasn't killed the Box Office. It may eat in to it, but the desire to see "anything" will never replace the desire to see "something that's actually worth seeing." If "professional" Movie Makers stop producing "things that are actually worth seeing", then youtube will win. But it will be because the pro's let them.
Not to offend anyone here, but there's a difference music I'd pay for and music I wouldn't... and no amount of music I wouldn't or don't have to pay for will replace music that I would and do pay for.
Does that make sense?