Why Bungie, why would you do this?! - Halo Community
I give up trying to change and argue against series that simply want to dumb themselves down to nothing. Its just easier to pretend Halo 3 ended the series and now there is new series trying to make their own failed version of Halo called lolHalo.
Besides, once too many games go casual, some new gaming company will see that, and will start making hardcore/dedicated games for the more "hardcore" audience that don't have many games. Simple supply/demand logic.
Arena shooters have gone down the drain I guess, but there are plenty of other genres you can play. Dedicated players can usually cling to a good RTS game. While most of them are on PC, Xbox has a few, like Halo Wars, which was OK in multiplayer and really good in single player. There are many entertaining new RPG's. XCOM is -blam!- amazing.
Overall, I believe that it isn't exactly "casuals" doing the damage, but devs afraid of a little risk. (New IP's might need to be careful, but your handed the keys to deving Halo...)