My understanding is that Bitcoin would fail if the energy costs would be low, the huge costs are what protects it from an adversary acquiring >50% of the "votin

My understanding is that Bitcoin would fail if the energy costs would be low, the huge costs are what protects it from an adversary acquiring >50% of the "voting rights" on what transactions are valid. At some point the mining rewards will stop, and transaction fees will be the only reward. The price will be driven by demand, but will need to be kept high.