I suppose what I think about is: What would we lose if said franchise stopped?
In CoD's case: Not much. There's no shortage of Multiplayer FPS games, and certainly no shortage of modern military shooters. There are so many CoD imitators, reminding me that CoD started life as a Medal of Honor imitator.
Unless they wanted to do something more interesting with WW2, like exploring some of the less famous parts of the war, then there's not much else for them left to say.
With Sonic, we'd be losing one of the more colorful franchises still in existence. Even when it's made failed attempts at being serious and dramatic, it's still proud of the fact that it LOOKS like a video game. Even with a handful of awful games in the series, it's still Sonic the Hedgehog and it's an important part of video game culture both historically and in image. CoD is basically a dull Michael Bay film for X-Box live trolls.
I'm not saying that it didn't have major impact on the culture, I'm just saying it's impact might not have been the most positive thing to have happened.
Besides, Generations and Colors have thoroughly proved Sega's ability to produce decent Sonic titles when they put their mind in the right place.
CoD, on the other hand, will be exactly the same the next time you play as the last time you played it. It will innovate only when it absolutely has to, and will never stop trying to screw you over on DLC and other shady practices, because that's what Activision does.