Halo 4 to Halo 5 was a three year time stamp. Halo 5 to Halo Infinite was a Five year time stamp.Keep in mind: Halo Wars 2 came out taking some 343 resources, MCC PC also takes some of 343's resources and a large amount of time needs to be put into making a game engine. While I am hoping Infinite will have at least some more content compared to its predecessors, it is still important to keep your expectations in check. Don't go thinking this is going to be the most jam-packed Halo experience because of the longer development time.
MCC, Halo wars 2, and Halo Infinite were all made from three separate individual teams. They didnt split their focus through the five years. So either way, the team that focused on Infinite had five years to prepare and surprise everyone.
Not saying that the development time is guarenteeing a great game, just that they had all the the team had a lot of time to make one.