My response would have been redundant with what I already told michaelt: your point is wrong, waiting <1 second for a visa transaction does not protect the merchant from potential massive losses, because of credit card chargeback fraud which is very real, exploited every day, and can be conducted up to 60 days after the purchase. Example: https://www.reddit.com/r/legaladvice/comments/5r9nqi/credit_...
"Not the same thing at all. People enjoy looking at colourful lights, and so choose to pay to display them"
And people enjoy making money, and so choose to pay the electricity to mine bitcoins. Same thing. People pay for things they enjoy.
"In a hypothetical large-scale Bitcoin economy, how many transactions [...], what proportion [...], and what would be [...]?"
I estimate that if Bitcoin's market capitalization reaches $1 trillion, then miners will still not account for more than 0.74% of the energy consumed by the world ($1.25 billion per TWh), which is more energy-efficient than, say, Canada (whose economy produces $450 million per TWh.) See http://blog.zorinaq.com/bitcoin-mining-is-not-wasteful/
"We have techniques for doing irreversible transactions in the existing financial system. They're used for a tiny proportion of transactions..."
Yes and this is a physical exchange of cash/commodities. Bitcoin/cryptocurrencies make irreversible transactions possible digitally and have no other equivalent in the digital world. This is one of the biggest innovations they bring to the table.
"the overwhelming majority of the time reversibility is what you want."
No. Litterally 1 in 2 persons conducting transactions would prefer irreversibility. Recipients want it. Senders don't.