From what I've noticed, game franchises are never intentionally ended. For example, Bungie never said "we're going to stop at Halo 3 and be done." (you can argue it wasn't their call; I'm not talking about that though) Franchises end after a number of unnecessary sequels push the game's formula and gameplay to its limits and it becomes very "watered down," and as a result the community becomes alienated from the franchise. After that, everybody moves on to other games, and the developer of the watered down game series begins to lose money and eventually end the franchise "prematurely," i.e. before the storyline they have made ends.
For example, Halo 4 (to many) seems to have alienated a lot of the original Halo community, and much of the community is moving on to Destiny. Just imagine if no one bought Halo 5 because it no longer was interesting in the face of Destiny's gameplay (which has a big chance of happening, I fear). Perhaps 343 would be forced to end the franchise before Halo 6 (that's what I mean by prematurely). Maybe it would have been better if the franchise just ended on 3, with a more definite conclusion. And this is just an example, I'm not saying this is how it really is.
Look at Gears: Judgment though. It's a prequel, and notice its stretching the boundaries of what the original Gears fans liked in the original trilogy. It has lower ratings than any of the other Gears games, most likely because it is "watered down." This is what I'm talking about. And this even goes for movies too. The Star Wars prequels. You know what I mean.
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[quote]Maybe it would have been better if the franchise just ended on 3, with a more.[/quote] If it was up to me I would have ended the Chiefs story at 4 and do something with the Arbiter for the next game. Once I finish both those characters I would be done with them. Then after that I would do Halo verson of Star Wars Battlefront and Battlefield spinoff to see how it works. If it works out I would do another game of it until it gets to the point where the next game might as well be DLC.