Hmmm... I love Bluegrass and I don't get tired of hearing it or the instruments that are used to make it. For the three days per year of RockyGrass I never find myself wishing for "something different" or a different style of music. I guess people who don't know what Bluegrass really is probably get tired of the terrible noise that a mandolin and banjo make and need to go listen to Bieber back at their campsite.
I want RockyGrass to remain a Bluegrass festival. I don't think it needs any clowns onstage playing dub-step or hip-hop. Look at it this way: there aren't a lot of Bluegrass musicians making a ton of money and playing gigs at every venue in the world. Bluegrass is a small slice of music in a big world. Why nudge a Bluegrass band off stage (at a Bluegrass festival) in favor of someone playing a completely different style of music?
Every year I hear a lot of people complain when non-Bluegrass bands are on stage. Most people want more Bluegrass. I suppose there are plenty who just want music and don't care what kind it is. There are certainly plenty of people who just want food and don't care what kind it is or how well it's made, which is why fast food chains stay in business. I don't like fast-food music. I like Bluegrass.