This is yet to be proven. Is the bitcoin network really more costly than existing payment systems? Can the security benefits, instantaneous transmission, and other features be replaced by less costly alternatives?
As the mining reward rapidly dwindles relative to the cost of electricity, there is a huge incentive to reduce the electricity consumption while maintaining the security of the network. At what point does this reach an equilibrium depends largely on the market value of bitcoin. So it should be a somewhat fixed cost in proportion to the adoption of bitcoin, just like the fixed costs of any currency (e.g. minting coins, mining gold, etc.).
As the mining reward rapidly dwindles relative to the cost of electricity, there is a huge incentive to reduce the electricity consumption while maintaining the security of the network. At what point does this reach an equilibrium depends largely on the market value of bitcoin. So it should be a somewhat fixed cost in proportion to the adoption of bitcoin, just like the fixed costs of any currency (e.g. minting coins, mining gold, etc.).