Let me put you on game real quick.. (This is all a theory)..I had to think about it but Microsoft's managers are GENIUSES....lol Just follow my thought process on this. Okay, the news comes out about the 20GB day one (Patch/installation/mulitplayerpart) download, almost 22 days before the release. Plenty of time to DIGITIALLY buy the game. This is a two fold strategy because it saves Microsoft money (no need to make an extra disc to ship with the physical copy) and cuts the resellers completely out (digital). The customer right now has an extra incentive to buy the digital version! You save time and you will be able to USE your purchase as soon as they make it available on the release date.
So my theory is that they aimed to have this update from the very beginning to maximize profit. They tried from the beginning to get rid of physical media, now, this is their chance. The next part of this to sell you extra memory. I don't know how they are going to do this but the size of these games are getting pretty big for a non-removable hard drive. They have to find out how to make money off your needs for memory next! The best part about this move is that they made sure the Titanfall part of Halo (Multiplayer) is what you have to wait for. They KNOW their customers want to get on that MP! So, get the digital version...or just wait. LOL
Damn you are tricky Microsoft....lol