Lets make this simple. I say: Time + Space = non-trivial investment. And further, I say: non-trivial investment = a value.
( Time + Space = Space-Time ) ∧ ( For all A, and for all B, in the universe of discourse, if A is an investment and A is non-trivial then it is not prudent to attempt an assault on PoW-coin B of investment A with A. )