it isnt obvious at all becasue the little bit of party games that have been released on Wii U havent been doing too hot. Very non Ninty games on the system have been doing well and thats kind of the point.
I'll take it one step further and say this whole genre and style is kinda dead and im not quite sure why they are trying to bring it back. It wont do great anyhwere.
I actually think Rock Band 4 will do well for Harmonix, there are lots of people such as myself who will be getting it just to bring my playlist over to the PS4 (or XB1). I currently keep my PS3 just for Rock Band because I do still enjoy playing the game.
The way Harmonix are doing it is perfect. A single release is all the generation needs, and then all the extra content via the store. That was one thing I really preferred with Rock Band last gen, porting your tracks out to the sequel. But yeah, why bother with a sequel at all really? (That was more an EA requirement I think, I'm sure Harmonix themselves would have been happy just rolling it all out as DLC)
I don't think the genre will ever hit it's lofty heights of 2007-2009 again, but there will definitely be an active enough userbase (and likely some newcomers) to make it worth while for a small independent studio.
2015 is looking like a fantastic year for Harmonix, what with Amplitude's reboot also coming.