To say that one transaction costs that much power is a misunderstanding of what the power is useful for. The amount of electricity spent mining a block is a function of the block reward, which is a function of the value of the total system. A block doesn't just protect the transactions inside of it, it also protects every single transaction in the whole history of bitcoin.

And, the high value transactions tend to dictate the value of the whole system. Not every transaction is equal in that regard. If all the high value transactions were to move to another system, the coin price would plummet, and so would the amount of energy required to mine blocks, and thus the 'per transaction' cost for low value transactions would plummet.

The right way to think about it is that we spend around a billion dollars per year protecting $70 billion in assets. Put that way, it doesn't seem so bad.

If you look at the percent of the US GDP spent on military defense, you'll see that bitcoin is quite attractive by comparison.