Yaya could always drop back but he has been impressive in his new found role. And if it is to be lined up like that only one o Tevez/Dzeko/Aguero/Balotelli could play regularly. they have depth that's for sure, but too much depth can be a badthing.
Just think about it, Mesut Ozil, who was/is better and more consistent than Nasri went for 15 mil euros last summer being in the same contract situation as Nasri, while Nasri who did really well for the first half of last season goes for around 22 mil pounds. Amazing deal for Arsenal but it does leave us very short in the middle.