I always regarded GH was for those who wanted a challenge and RB to be for those who just wanted to have fun. Fun, remember that? Before you went online to get the online achievements in GHSH and the only song anyone played was TTFAF?
Was a whole lot done from GH5 to GH6? Going from 3 to 4 included vocals and drums, 5... didn't do a whole lot, and I suspect 6 didn't do a whole lot either. Metallica had a soundtrack that was just over half Metallica, Van Halen actually took a step backwards by not using the concurrently released GH5 engine. (BTW, Metallica and Van Halen vans should be thanking the Aerosmith fans who had to put up with a really suboptimal game so they could get better games.) Oh yeah, and then there was Band Hero which didn't do a whole lot of anything.
RB1 to 2 improved the Tour mode and added drum solos, and 3 added keys and Pro mode, which I'd take over the entirety of 2112 any day. Beatles added vocal harmonies, but Green Day didn't do a whole lot either. This isn't to say Harmonix can't screw up RB, but at least they're not oversaturating the market... yet. Hopefully they'll learn from GH's errors.