[Detrytus]

Isn't the whole point of PoW that the work done is otherwise useless? I mean: you invest money (in form of your hardware/electricity bill) to mine a block, and that ensures that whoever would like to fork the chain has to spend at least as much money to do it. If PoW can earn you extra money outside of the Bitcoin ecosystem (by making the work "useful") it lowers the cost of the 51% attack, potentially making it profitable.

[wslh]

I understand that there was a problem finding a useful PoW from the computer science perspective, and that paper is showing a solution.

Now, it doesn't necessarily lower the cost of an attack because you can adjust the required output to a cost that is suitable.