A safe blockchain requires enough power so that no other entity can reproduce what you've done. It solves the Byzantine General's problem so you no longer have to have trust in anyone that the transaction is real.

But it only works if the difficulty level is high enough that a determined competitor can't split the chain at a lower block, reproduce everything you've done with changes, and outrace you to build an even longer chain. In Proof of Work blockchain systems, the chain showing the most work is always true.

By dedicating enough energy, the Bitcoin blockchain becomes immutable, because no other country/group will have the energy to wipe the history of what has already been written.

It comes down to who you trust more. A banker, or the math that underpins the blockchain? It may take less energy to trust a banker, but I'll trust math any day of the week.

No matter what you think of currency and the economy, Bitcoin is needed because there simply is no other mechanism available to man that carries this level of certainty. If we give up on it, and don't build a few nuclear reactors to keep up the difficulty level, we're back to trusting in our overlords again.