Why stop at Halo 3? It was nice to see the fall of Reach and Earth through video games. Halo Wars however was bad.
MW should have stopped at MW1, who would want to buy the same game three times?
Well I got all three(I never bought MW3, it was bought for me), and had lots of fun with each one and each one did something a little different. I don't want them to change the formula too much for multiplayer. Nor do most of their fans. Small changes and basic improvements though? Yes. Which is what they have been doing. Though the yearly release thing is bogus. Each game releases every two years. There are two dev teams working on each CoD game. I am not even a big CoD fan. The internet rage at them is just hilarious though.
As for Halo, well Halo 3's Legendary Ending indicated his story was to continue and if you payed any attention to the other parts of the story there was quite a lot of stuff that was unresolved and given the books a 4th game was inevitable. Btw, Halo Wars was fun for some people...not the hest RTS but it was decent for a console RTS.
I am mostly playing devil's advocate here though. I think CoD should release one of it's installments every two years. Meaning each dev spends 3-4 years on development. They pump them out far too quickly.