But considering the fact that the computations are useless, the energy is still wasted.
I don't think Visa uses up as much energy as a small country for processing 7 transactions per second at full speed.
(Eg even with bitcoins, you'd still have essentially the same stock market. You might be trading coloured coins on the blockchain, but people would still need to work to raise capital etc.)
I guess where we disagree is in how much of a proportion effort in finance is spent on producing that kind of trust vs other activities (like eg capital allocation, or hunting for arbitrage activities etc).
This "waste" is actually a kind of security all its own.