Giving up and moving on are two completely separate things. After ten years, as a developer you might want to try new things. Developing for a series like Halo, you have your balls in the vice of the fanbase; go too far off from the core of the series and you'll be singing only the high notes to your favorite songs. They wanted to do different stuff so they had to make a new IP for that; in fact it gives a breeze of fresh air to the industry. If this were ever the case, as unlikely as it is, there probably isn't much preventing them from making a new Halo if they so choose. I'm sure Microsoft and 343i would jump at the chance to get Bungie involved in a new project. Spartan Assault proves that they're willing to let other developers make games for the series.