BitTorrent is centralized. Everyone has to point to the same torrent file, which has to be stored somewhere.
P2P is about redundancy. However unecessary redundancy is waste, which is why networks centralize in the first place.
Napster centralized search because search works best as a centralized function. Only when legal troubles cropped up did it become worth it to decentralize search. Napster never centralized storage because storage was hard(legally, economically, bandwidthwise) to centralize.
Cellphone networks are distributed because the problem of Coverage is best handled in that manner. However the problem of Continuous Connectivity requires a central coordinator. In the absense of a central coordinator, the individual devices must have, and maintain, the intellect to route their own messages.
Decentralization is inevitable, as the economics of information change. P2P, however, is not inevitable. In fact it is usually only the answer when there are legal,economic, or physical constraints to control.