I've heard several people bring this up (and using the very same examples of Pirates and the Matrix) but if anything all it proves is that big-budget cg-heavy sequels make TONS of money, be they terrible or not, and keeps fans coming back--wanting more. The Matrix sequels? Terrible yet made $1,173,071,759 (not including Animatrix). Pirates sequels? Garbage and made $3,069,110,875. Transformers sequels? Megasucked and made $1,953,779,467. See a trend? If not, look also to the Jurassic Park, The Mummy, and Star Wars sequels and prequels for reference.
Also, look to trilogies like Indiana Jones, Toy Story, Potter, original Star Wars, etc. for instances where both money is made and the films were GOOD. I think Avatar is a win win for Disney either way. It just translates well into a theme park whether or not the film was great (much like TRON would).