It doesn't matter if the work is profitable because you only need to create an alternate history for a few blocks.
If you attack the network you can steal billions - so if you have that much computing power available for your useful work, you can temporarially use it against the block chain.
As long as that mining power is available anywhere the chain isn't secure.
If you attack the network you can steal billions - so if you have that much computing power available for your useful work, you can temporarially use it against the block chain.
As long as that mining power is available anywhere the chain isn't secure.