Unless everybody moves to freenet (or some other friend-to-friend network) you have to exchange keys at the beginning of every session with a peer. The ISP can

Unless everybody moves to freenet (or some other friend-to-friend network) you have to exchange keys at the beginning of every session with a peer. The ISP can see that, and MITM you. SSL/TLS gets around this with a central certificate authority that vouches for the public key of the other party. That's damn difficult if not impossible to pull off in a decentralized system.