Jeremy’s Evil Twin has a point–I don’t think it applies, but it’s a point nonetheless. 
The generalization, “Historically, fantasy movies do not do well” is true, in a narrow sense. However, it’s not a very good guide to the success of Harry Potter or Lord of the Rings.
The first thing to remember is that audiences don’t know or care about these box office “rules.” If you had said in 1974, “Historically, science fiction and horror movies do not become blockbusters,” the box office pundits would all have agreed with you. That wouldn’t have stopped audiences from lining up for Star Wars and Jaws later in the decade. The public’s tastes change, as does Hollywood’s ability to bring off these kinds of movies. (Lord of the Rings would have been impossible to film only a decade ago. Most earlier high fantasy movies like Willow and Legend had cheesy, unconvincing efects, even by the standards of the day.)
The generalization also defines fantasy so narrowly that it’s virtually meaningless. Never mind that Star Wars is fundamentally a fantasy story with “Jedi” hastily substituted for wizards and “light sabers” for swords. It only counts as “science fiction,” according to the pundits, not as fantasy. (Though I defy you to find any science in it.
) Likewise, Batman, Who Framed Roger Rabbit, ET, Mary Poppins, Crouching Tiger Hidden Dragon–none of these count as fantasy movies according to the “fantasy is box office poison” crowd. I don’t think audiences have such narrow genre tastes, personally. They just want to see something that’s really, really cool.
Last, no previous high-budget fantasy movie has had such a large pre-sold audience. There were literally millions of people clamoring to see these two movies before a single frame was shot or a single ad dollar spent. That gives these two flicks a huge head start.
True, these movies will probably “only” make $200-$400 million apiece. But they still have a better shot of breaking Titanic’s record than any movies released since '97–certainly better chances than the bland, imitative, over-hyped Pearl Harbor.
(I’m also perplexed by the Twin’s claim that Lord of the Rings lacks the older demographic. Tolkien was practically the bible for the entire '60s generation!)