it's all about the money, yes, but if you knew anything about Hollywood logic these day, you'd know that's it's all about the trilogies now in days.
lord of the rings. very successful.
the matrix. fairly successful.
the new star wars. big.
pirate of the ****ing carribean. huge.
the upcoming batman trilogy. great, and will be successful.
shreck the third (possible end to a trilogy). will be huge.
they've cashed in already. they know that. and the cash will keep coming in even after SM3 is released. there are so many ways to keep the cash flowing.
if they're smart, they'll learn the the WB's mistakes, complete this trilogy, and leave the door open for the future. you wonder why they'd stop while it's hot. i say they should go out on top, rather than possibly risk making anything worse than what we've got.