Why on earth would you halve the rewards? Belay that and focus on security. Someone with either epic hashrate (more than all alt's combined) is toying with the coin, or it's someone attacking the network itself.
I agree...halving the rewards could cripple the coin.
The miners who stuck around all of a sudden get half the reward they are used to while the potential attacker walks away with a nice big sum of coins. And nothing to stop him from doing it again.