Bitcoin's mining reward halves every couple of years. When the Bitcoin block reward becomes negligible, as it certainly will in within a few decades, the only thing funding Bitcoin miners will be transaction fees, so the entire cost of Bitcoin mining will be borne directly by users of the network.
The amount of money spent on electricity for mining is exactly the same as the (perceived) value of the mining rewards. Therefore, in the future the amount of money spent on electricity for mining will certainly go down as a percentage of Bitcoin's value.
If Bitcoin becomes very popular and valuable, transaction fees may be significant in absolute terms, but they will always be a very small percentage of the total, and likely smaller than the percentage of economic output consumed by the traditional financial system (in the future when the block reward is small).