The best way I've seen to think about it is that the difference between GHTunes and RBN is like the difference between playing Rock Band and playing a real instrument. One's a accessible and fun diversion but you generally can't get too much out of it besides entertainment, while the other requires a lot of skill, time, and money to use but is a much bigger deal and you can potentially profit from it.
In short, Rock Band Network doesn't obsolete the GH music creator for the same reason that real instruments don't obsolete rhythm games.
The GH music creator mainly solves the first problem. It lets anyone make and play their own little songs without much musical knowledge or skill. The ability for other people to download them helps a bit for the second problem, but the low quality of the songs produced means most of the appeal generally relies on the ability to create your own, not to download other people's songs.
RBN exists primarilly to solve the limited DLC problem. By allowing bands to do the work themselves (or hiring someone else to do it), there will be more songs available as DLC than Harmonix could possibly chart themselves, including from bands that Harmonix has probably never even heard of. It may also satisfy the hardcore custom charters a bit, but the high barrier of entry for creating songs (money, lots of time, relatively steep learning curve, needs to be professional quality, and you need to actually have the rights to a real song) means that it's primary purpose is to increase the amount of DLC available and the ease with which bands can get into Rock Band.