Bitcoin miners have no incentive to mine Bitcoins at a loss (relative to electricity cost). Rationally, each miner would shut off if the price goes below this point. (They might be in a position of having spent money on hardware that they can never hope to recoup, but still be operating above their electricity cost, as a way to reduce losses).
Large NXT holders should be willing to forge, even at a loss, because they risk losing all of the value of their stake if the network dies. Unless they give up completely, they should be willing to continue forging. Its not altruism, it is in their best interest to keep the network running so that they can hope to recoup the value of their stake in the future!