I don't think the fact that they are guaranteed a box office of $100 M + would be reason enough alone for a studio to green light a sequel to an enormously successful film.  There would have to be artisitic merit to the script proposal and some important character and narrative advances that they felt the director could bring to the project.

Admit it, I had you going for a second there.

The first ones made plenty of money, so of course they'll make sequels, which won't be as good as the first ones, but people will watch them, so they'll still make money. If the sequels make enough money, then they'll make a third one. If there was enough money to be made there, a fourth direct to DVD movie is always another possibility. They'll suck every last penny out of any popular movie.

Gee, this would be a real shock.....