Thank you, this is a very productive reply, and you are highlighting specific things that are unique to Bitcoin.
You're right that a core part of its design is making sure real computational resources are spent on it. That means the (total, social) cost network as such will scale with the size of the economy, not the number of transactions. So, in a sense, there's (something that looks) like an arms race going on, where more resources spent on it don't mean more transactions. Point taken.
But that doesn't mean there's zero net benefit. The benefit is a permissionless network that no one can control unless they produce more computation than the rest of the network. So, the benefit of each miner coming on is raising the attack threshold and increasing your confidence that it won't be attacked. At equilibrium, the cost will be some fraction of the economy, not all available growth or energy (like a gas). That amount of energy that this involves will decrease as energy is taxed.
That's pretty much the dynamic of gold mining under a gold standard: the larger the economy, the more money being spent mining (literal) money.
I agree that it's a high price to pay, but I don't think the benefits should count as zero. Being able to bypass financial choke points so you can donate to Wikileaks is definitely a positive.