Why are producers of the Terminator franchise obsessed with the idea of a trilogy?

When Terminator: Salvation was launched we were told this was the first in a trilogy that would cover the man vs. machine war talked about in the original movies. Of course those plans quickly fell by the wayside after the film's so so box office returns. The same line was trotted out with the release of Terminator: Genisys, the movie would be the first in a new trilogy that would reinvent the franchise. 

We've known for some time now that James Cameron is planning on producing a new Terminator movie with Deadpool's Tim Miller in the directing chair. The latest word on the plans comes from News Australia where Cameron is quoted as saying that he and his producing partner are contemplating a, you guessed it, new trilogy of Terminator films. 

"I am in discussions with David Ellison, who is the current rights holder globally for the Terminator franchise and the rights in the US market revert to me under US copyright law in a year and a half so he and I are talking about what we can do. Right now we are leaning toward doing a three-film arc and reinventing it.”

In the same interview, Cameron said that he wasn't a fan of any of the Terminator films that were made after his departure from the franchise. 

“I think it’s fairly widely known that I don’t have a lot of respect for the films that were made later,”

Well actually Jim, there is this Terminator: Genisys featurette in which you were quite complimentary about the film. 

Though to explain that it seems he's pulled out the old 'supporting a mate' excuse. 

“I was supportive at the time in each case for Arnold’s sake because he is a close friend. He has been a mate of mine since 33 years ago so I was always supportive and never too negative. But they didn’t work for me for various reasons.”

They didn't work for a lot of people in fairness. It will be interesting to see how Cameron and co go about reinventing the franchise. As he says himself in the same interview, can the franchise really have relevance now in a world that has almost caught up to the science fiction of the first two movies?