OK, so we're all a bunch of serious gamers here. Many of us are even hardcore gamers. And we all fall somewhere on the scale of difficulty where the low end is autopilot and the high end is the 3-ring binder version of Falcon 4.0 or some of the heavier Derek Smart titles. And so many of us are going to have a big ol' shitfit over the mere concept of a "hit one button over and over" game. And on cue at least one or two people will take the hypochondriac route of proclaiming a fear that one day in the not too distant future all games will be this way and we must act now to stop it, etc.
But consider this - my wife and I have another couple we're friends with. This couple is older than us but not old enough to be considered another generation. Neither of them have owned a game console since the NES era and while they own a PC or two capable of playing, say, PopCap games, they generally stay away from the PC in the evening hours since to them, sitting in front of a PC is what they do for work all day and they can't imagine why you would want to do that once you get home.
And like most people who fit this description they came over to our house, saw how much fun the Wii is, and bought one as soon as Costco had a bundle for 30 minutes. The last time I went over to their house, they had about 15 Wii games. And while their taste is questionable, it is interesting to see what "non-gamers" buy.
One of the games that they went out and bought Guitar Hero: World Tour, specifically the version that comes with two guitars. They play it a lot and my wife liked it so much she wanted to get it herself. I was able to convince her that Rock Band is the better franchise and so we bought Rock Band 2 for the Wii.
When we saw our friends play GHWT on the Wii, the wife in the pair was playing the "easy mode". I'm not sure when this started but whereas "Easy Mode" in Rock Band games is basically three notes, "Easy Mode" in GHWT is... whatever note you want. Like instead of the little buttons coming down the fret, it's solid lines. Any button you hit, so long as you strum at the right time, you've got it. My wife's first complaint about RB2 was that it was "way harder" since there wasn't this simple mode to start with. But she got over it and now she's moved on to medium.
This was last Christmas. We recently played RB2 at our house with the same couple friend and we discovered - the wife in that couple still plays on easy mode. She's tried medium or whatever the next one up is but she can't do it. And she doesn't want to do it. She's perfectly fine with the rainbow road mode. To her, the fun is in playing the game, not challenging yourself to a new level.
I don't think the super easy mode of GHWT is why that series sells so much more than the RB games (that would be name brand recognition). But I do think there's an entire section of people who aren't typical "gamers", but like to play in their simple way. These people are the target audience for the "just hit the button over and over" games. They're not hardcore, they're having fun, and they're spending money. So long as it winds up being a "super easy mode" that doesn't detract from gameplay design and is purely an option which people like us will skip, what's the harm?