I'll add my own take on what's already been said and said well by many earlier posters, but the huge mistake here is thinking that somehow the same people who love themselves some Gears of War 2 or spend years playing World of Warcraft were really just dying to do something really exciting like play Farmville. Do some people who play those games -also- play Zynga games? Well sure, there are some folks who game 24/7 and might dabble in a little bit of everything, but that's not really who we're talking about here. If you switch the sort of media you're talking about, it immediately becomes more clear as to how ridiculous a concept this is.
With the success of Harry Potter, for instance, did publishers scrap their romance novels and techno thrillers? No. Now, did some authors decide to write more children's fantasy? Yes. What that did was create a bigger market, but it didn't change the fact that there were still all those people who liked romances, mysteries and thrillers that had absolutely no interest in children's fantasy books. Did some of those voracious readers also read Harry Potter? Of course. Did that mean they stopped wanting to read their Anita Blake novels? No. And heck, modern horror, which is often modern erotic horror, is really darn popular too, but does that mean that's all anyone wants to read? Hardly.
I picked on fiction because it's the other media I'm most familiar with, but I'm sure you can apply it across all of them. New, fresh ideas come along and everyone wants to make money off them, but it seems to be a lesson learned anew each time that you can't entirely abandon what came before with the idea that somehow this shiny new concept changes everything. The question is whether or not EA or Activision should waste resources pursuing this or whether they should, perhaps, buy or form some smaller development studio to do little casual games.
Is there a chance for huge ROI based of cheap casual games, on Facebook, phones or otherwise? Sure. But does that mean that people are going to stop being interested in Modern Warfare and Madden to go play Angry Birds and Farmville? Not a chance.