I don't think that's the primary culprit.
Competition has caught up. "FPS" as a genre has never been more popular. If CoD was still the only big player in the market, I would not expect to see such declines.
But games like Battlefield and Far Cry and even Titanfall and Destiny have redefine what an FPS can be, with the production values and marketing budgets to compete whereas CoD had that mountain to itself in the past.
Those games have such long retention plans built into them, too, splitting a playerbase.
I don't think annualisation is the #1 cause, in fact I actually think annualisatoin ultimate grows your brand. When managed correctly, you keep people engaged longer and you increase ARPU through various means that you don't even lose revenue off a declining initial sales base.
But anyway. Competition is good. There's so much money to be had there.