I'm always pleased to remember that in the 1950s when I saw teenagers on every street corner singing in harmony and playing music (usually very badly), I knew I was viewing the start of a big trend. In the 1960s and 70s, when the world was full of skilled rock groups, I knew where they had come from. So now, let's compare todays' Mashup artists to those rockers.
The Web is full of clever artists who photoshop images together, or superimpose audio tracks, or combine and modify other art to make derivative videos. Sometimes they do all three to published works, to create a new thing of beauty. And the big music and video companies go right after them, suing to make them put their derivative art away.
In ten or fifteen years, some of today's Mashup artists will occupy positions of power and influence. I have no idea what they'll actually do, but I wonder if they'll want to get even? (A little perspective – until about 1930, most of the world assumed that modifying other artists' art was a reputable thing for an artist to do. Until about 1900, Western Europe and the Americas acted the same way.)