I really think its hard to answer this question because hardcore gamers come in a million different categories.
I have friends who play hours of CoD every night, and a few of them are really interested in the new Zelda.
They are the gamers who play a wide variety of games and usually own a few different gaming systems.
Then there are those who don't care, because they only like shooters, or they only own an Xbox, or are only interested in M rated games, or have just never been interested in Zelda.
People who will probably buy it:
Someone who has played or owns a Zelda game and liked it a lot,
Zelda fanatics who do/don't but will own a Wii,
Gamers who like action adventure games, and own a Wii,
Younger kids who want a good game for their Wii.
One of the seven million people who own Twilight Princess and really liked it.
XBOX, PS3 fans who don't own a Wii will not get it. Its really that simple. If they do own a Wii, then its totally up to their gaming preferences.
The M rated fanboys will be turned off by the bloodless cartoon graphics, but they probably wouldn't give it a second glance unless it was super realistic with lots of heavy sword combat and looked all Lord of the Rings-like.
Conclusion, this game probably won't sell as well as Twilight Princess because it's cartoony. A huge chunk of gamers love their violence, and a lot of people bought Twilight Princess because it was "serious" and "mature" looking. Fortunately, Twilight Princess opened a lot of gamers eyes to the glory that is Zelda, even if its cartoony.
I feel it will do better than Majora's Mask and Wind Waker, because of the success of Twilight Princess. I don't think it will do as well as Twilight Princess though. I'm gonna say around 5 million units sold. 2 million less than Twilight princess but 2 million more than Wind Waker.