I think you underestimate and/or understate how big of a deal Bitcoin's lack of reliance on a trusted third party is. That's essentially the entire point of Bitcoin, so it seems a bit disingenuous to call it "wasteful." Perhaps if you have no desire for a decentralized transaction log with no trusted third parties, then it would be wasteful for you to throw computing resources at Bitcoin, but it's ridiculous to apply that generally.
Still, think about how the proof of work operates. There is no connection between the amount of computation needed to prevent attacks and the current block reward. Therefore logically the amount being spent on mining is very probably far too high or far too low. It's possible that it's too low, and bitcoin could be taken out by a government body. I personally think it's more likely to be too high. As in, X attack only needs to cost $1M to keep the network safe, but the current mass of miners makes it take $10M. The other 9 million is truly wasted on the tragedy of the commons.
To go back to the silly analogy, you need a 20 ton lock but you can only use 'cost plus' bidding and all the contractors keep making the lock bigger until they get every possible cent out of the process.
