Mining for Bitcoin produces something that can be sold to speculators or be used to pay for drugs and assassinations.
And most of that mining (~65%) is in China where 65% of the electricity production is with coal.
What do you do with unused hydro output? You can waste it or you can power bitcoin. This could be arbitrary big numbers in terms of KWh, but the opportunity cost and carbon footprint would be zero.
Since bitcoin mining is so perfectly competitive, it will be done where the cost of energy is lowest, which is zero for the most places - where energy would be otherwise wasted. And since other consumers pay considerably more than zero, they'd get oil/gas/coal delivered to them instead of wasting them on bitcoin miners.
You are forgetting (among many) about the impacts of a decentralised currency for the billions of people without access to bank accounts or a stable currency.
Global average 1KWh = USD ~$0.14
621KWh * $0.14KWh = ~$85
That sounds like a lot, but it's about $10 worth of electricity for the miners. Another comparison might be that that's about how much electricity it takes to smelt 4kg of aluminum. I don't think it's meaningful to think in terms of "waste" wrt energy consumption any more than gold mining is "waste". Most of the energy generating capacity would not have been turned on if it wasn't going to be used for mining bitcoin.
The only useful framing is in terms of externalities, but it turns out the cheapest forms of energy generation (underutilised hydro and solar in china) tend to be zero carbon. Using energy isn't bad, emitting carbon is bad.