It's not all that easy or cheap to decentralize the last mile. Things were not any more decentralized when most people used dialup, and they were definitely more centralized when the main users of the internet were on a few campuses.

Absolutely true. Getting a phone tap and the equipment required to monitor an active modem connection was sufficiently onerous for local and federal law enforcement that it was reserved for active investigations of high value targets. Hell, they couldn't even be arsed to track Mitnick down when he was on the Most Wanted list, it took a phone company tech and a pissed off security analyst from California to finally bring him down.

Decentralized does not imply secure. Obviously both are desirable, but one does not imply the other. Although some will (rightfully) argue that a centralized system can never be fully secure, so you might say that secure implies decentralized at some level.

Although, to be fair, of all the above only (the old) Skype and email come close to decentralization.