This is not death of COD. Far from it.
Customers did not find MW3 as appealing as BO - therefore, reduced sales in the longer period relatively.
Maintaining momentum is always a challenging scenario in an intensely competitive environment.
Hopefully, COD developers will now pay more attention to importance of innovation and customer demands.
And Yes! Gaming industry does not ends at COD. Many people try different games all the time and develop new likings.