Augusto, I agree with you that it is true that a trustless transaction system can exist in any type of p2p network, including one built with human trust institutions (like SSL).  However, there are many reasons why a human "trustless" p2p network is preferred over one built on human trust institutions. 

More accurately, some of us would like to preserve to the fullest extent possible:

- transaction user anonymity

- node anonymity

- global network accessibility (keep governments from being able to block access)

- network uptime (keep anyone from shutting down honest nodes)

- secure (not easily compromised just because a human institution or its data become compromised)

It's clear that some people have no regard for anonymity, some value it a little, but not essential, and many regard it as essential to protecting our freedom in world where technology is increasingly concentrating central power -- and power corrupts.  So, once a person is identified as being in the first two camps, the rest of us, well, aren't likely to trust them. 

The great thing is we can use the very technology used to create bitcoins, including the combination of the p2p network and the blockchain, to create new concepts that strengthen the network without throwing privacy to the wind. 

While node trustability is distinct from the transactions in the blockchain, the blockchain is in effect our first large scale p2p database.  This type of database depends on the survival of the p2p network.  Likewise, the p2p network can better survive if it is extended to leverage not only the bitcoin blockchain, but new blockchains that can hold new types of information.  Perhaps the next evolution of a highly resilient self-healing p2p network is one with its own non-currency blockchain(s) so it can learn, build intelligence, and respond intelligently to threats.