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!
OK I can follow that logic, thanks. Next (no pun intended), lets look at network security on a per confirmation basis.
With bitcoin I can easily estimate the cost of reversing (by renting hash) a singly confirmed transaction: 25.15 BTC (25 coinbase plus average of 0.15 fees).
Now how much would a similar estimate run (renting NXT from holders to get the required stake) to reverse a singly confirmed NXT transaction?
2 confirmations 2x 25.15 BTC, 3x ... this is bekomming costly of people have some half hour of time to spend on security.
Staking coins usually have way faster cycle times. Even getting worser if you scale that up onto a half hour.