Bitcoin miners are utilizing excess energy that would otherwise have gone to waste. So there's nothing wasteful about it. In fact, miners must be as energy efficient as possible in order to remain competitive and profitable. Typically excess power is the cheapest source because there's no demand for it, hence why efficient miners use it. Check out great American mining[1] for example. They harvest wasted energy from gas flares to power bitcoin mining rigs.
This is basically false (some miners might do this but they're a minority). Almost all bitcoin mining happens in China, and happens in an area where a large fraction of grid power comes from coal. Using excess power doesn't work well for bitcoin mining because the power draw is constant: miners are generally not powered on or off over the coarse of a day: they're turned on and not turned off until they're decommissioned.
Even with free electricity, the embodied energy of the computers used is basically wasted. A simple notary service that signed transactions with a timestamp and published a record to a public data storage bucket could handle worldwide bitcoin transaction volume running on a single machine, if you traded proof-of-waste for a simple root trust store.
(I mean, if I wrote it it would probably take several hundred machines but I'm sure Satoshi or some other brilliant programmer could fit it on a single machine. It only processes what, 5 transactions per second? My first dumb cellphone might be able to do that many signatures per second if someone could figure out how to compile the relevant libraries to run on it.)
Personally, I prefer to use a variety of proven private ledgers with a variety of trust roots, independent audits, and insurance. Using a public ledger is not something I would invest in. I'm just saying Bitcoin is a wildly, incredibly inefficient public ledger service.
