Lowering costs will attract third-parties, but we probably won't see much of an increase in mature games. That's my guess.
As for the average user base, even if the GCN has 18 - 25 year olds as its average age group, remember, only 6 million GCNs are sold globally. Compare that to the massive amount of PS2s sold, and you can see why some developers choose PS2 to put their major games on. Also, Nintendo's games are what really sells on the Cube. Most people who buy a Cube probably have either a Mario, Zelda, or Metroid in their collection. Third parties try, but get shot down compared to Nintendo's games. Look at NBA Street, RE, Smuggler's Run, etc...they're all good port (especially the remake of RE), but most people would save their money for a Nintendo game.