But please do understand, I'm not speaking of a situation of two co-existent currencies. I'm speaking of the current block-chain being taken over.
Did you mention HOW the current block-chain would be taken over?
The only way this can happen is for a single individual (or group in collusion) to own more than 50% of the hashing power of the network, as that is the only way that forged blocks can be added to the legitimate block chain.
Even if that happens, it only gives you the power to double-spend your coins, as you can put new transactions in the block and then verify them yourself.
However, you have to also then decide which is more profitable, double spending your own coins, or making more, as with 51% of the hashing power of the network, you're also reaping 51% of the bitcoins being produced by adding legitimate blocks to the block chain if you stay legitimate.
Forged blocks can also be identified by the community by looking at previous blocks and showing that the coins were in fact spent twice. That will invalidate your impartial status, and will most likely result in the loss of miners to your pool, which will drop it below 50% of the hashing power of the network.
Also, new pools will also constantly come online, offering better reward methods to miners, which will result in miners leaving more established pools, thus further distributing the hashing power of the network.
