My guess is that the Wii is going to hit it big with the baby boomer generation, due to the more accessible gameplay of its titles. While the baby boomers are old enough that they do not tend to intersect the typical gaming market, their numbers do consist of a large segment of the US population (around one quarter of it by most accounts.) They're also heading towards retirement age, and will be spending a reasonable part of their retirement funds on entertainment. The timing is basically perfect for Nintendo, and a similar situation exists in Japan - albeit for a different reason.
If Nintendo could pull it off with the Wii Health Pack(or Wii Music), prepare to watch the Wii dominate at least as much as the DS is dominating now, especially in Japan.
What I think is much more crucial to the Wii's long-term success, and maintaining its momentum, us coming out with games that appeal to the mainstream non-gamer other than Wii Sports. When/where is the Wii's Nintendogs, Brain Age, etc? Does WarioWare count in this category? I'm sure Nintendo has to be cooking up an exercise/coordination game along the lines of Brain Age... those are what will solidify the Wii as a generation-dominator.
Pulling in the casuals and non-gamers is just phase one of Nintendo's strategy. They want to build the install base, and Nintendo believes that once the userbase is there, the software support will come. Remember, the Wii is being marketed as the console for everyone, including the hardcore gamers.
When/where is the Wii's Nintendogs, Brain Age, etc? Does WarioWare count in this category?
It's been packed in with the console since launch in North America and Europe, and is the number one reason why the Wii is still nowhere to be found in store shelves three months after launch, even after more than 4 million consoles have been shipped.
"Demo channel?" I highly doubt we'll see downloadable demos of Wii games: there's no the storage. OTOH, a "DS Demo" channel seems like a no-brainer, and would itself drive sales like mad.
I've heard that Nintendo said that external harddrives would be supported, and like baka said, the hardcores will eat it up, and it just so happens that they are also more likely to buy a third party external harddrive or an official Nintendo one.
The DS demo channel should be a great way to capitalize on the DS' success and I think Nintendo would be stupid enough to not do this.