I agree with you to a point. The dance music of today is Breakbeat, Dubstep, Grime etc (pretty much what's in the rinse arena at GG), whereas in my teens it was Garage, Drum and bass, Hardcore
and Hardhouse. However I know for a fact these genres of music are still going and are still really popular with the kidz.
I don't think these types of music are necessarily less popular, I just think that GG can see a gap in their festival business plan and can see extra $$$ being made by including a more mainstream lineup.
The underground artists are still there, it's just the headliners are there to pull in more people from the mainstream groups.
My younger bro (21) hasn't got a clue about dance music, his imagintation just about stretches to Pendulum. When he starts talking about going to GG I know it's sold out!