In practice it is better to build p2p systems under the stronger assumption that actors need not be economically rational. Otherwise the market could be attacked (say by a nation state that wants to kill it) by repeatedly building up rep, then scamming on a large transaction (even if the accumulated rep is worth more) to destroy faith in the market itself.
Of course. But if this is not possible, we're much better off with a system where at least actors cannot profit from being dishonest.
I think Bitcoin is a great example of a system that is not cryptographically secure per se, but still holds up because actors need to spend money to - temporarily - prevent it from functioning properly.
