Of course they're milking the series!
If you think about it, who wouldn't?
Keep pumping out sequels of one of the greatest shooters of all time, Bill Gates must be behind this...
Not necessarily. If you actually care to play the games you'll see that Bungie is decent enough to make very good games that aren't total failures and wastes of time. I'd almost consider Halo 3 ODST a sequel if it had a little bit longer campaign.