Just because a game was good a decade ago, doesn't mean it will be just as good now. Halo 3 is a lot of fun. I spent COUNTLESS hours playing it. I reached Brigadier Grade 3 and a highest level of 46. I was pretty dang good at it.
...but it's antiquated now. There is NO way that it could last in the industry today. If 343 made a game exactly like Halo 3, but just put some spit and polish on it...it would flop...HARD. They need to be able to compete with fast-paced titles such as Call of Duty and Battlefield. A Halo game without sprint, would flounder in the sea of FPS titles that do.
Going back and playing Halo 3 is fun. It's nostalgic and it is more than capable of producing even more memories and good times as ever before...but it had it's glory days. Let it go.