Except Pirates was gonna be a trilogy from the start.

I don't believe that.

Pirates feels like a completely stand alone film that was successful enough to spawn sequels. Pirates became a trilogy with "two part" sequels because that was the generic thing to do at the time

[Pirates] 4 and 5 have nothing to do with the others and had a different team behind.

I agree. I just mean Pirates 2&3 as part of a group consisting of Matrix Reloaded & Revolutions and Back to the Future 2 & 3. That being said, Pirates 1-5 provide a nice counterpoint to Avatar and highlights why I found the initial rumors of a basically pre-ordered Avatar 2-5 surprisingly bold.

Perhaps the best comparison to the Avatar sequels is Lord of the Rings (which, according to Vigo, had a lot more filming to do for Two Towers/Return of the King than was portrayed in the press). It's an ambitious self contained project and the studio has to invest a lot of money up front but they have space to decide if the second massive pool of money needs to be paid out.