Some statistics taken with a Kill-A-Watt meter from a 2010 Mac Pro with 2x X5760 Xeons (12C/24T total), plugged into a Dell 22" IPS display:
Plugged in, but off, with monitor in standby: 1W
Plugged in, off, monitor in standby, 60W LED on: 7W
On, in sleep mode, with monitor in standby: 5-6W
Monitor and Mac Pro on, typing this message: 150-160W
12C/24T loaded to 100%: 322W
Now, how much time do I spend in each of these states? By far, probably 90% of the time, the system is off or in sleep mode. Using the above, even an ancient, sleeping 2010 Mac Pro workstation consumes less than an LED bulb in the on state. Almost all of the remaining time would be on in a relatively idle state, so bouncing around the 160W mark. I'd say it's definitely way less than 1% of the year that I would even come close to that 322W max-state.
Now, I'm not a gamer anymore, but considering the state of gaming these days, I bet many gaming rigs are active every evening for hours on end, consuming way more than 322W. A premium GPU will gladly hit 300W, and top-end CPUs will climb well past 100W (an active PS4 Pro consumes around 150W, IIRC). Mining rigs with several GPUs running 24/7? That will consume more energy than my Mac Pro under my usage pattern for many many years. So what is actually harder on the environment, buying shiny new energy-efficient machines, which bore a significant environmental cost to produce and ship to your store or front door, or simply using something that already exists? More and more renewable energy is going into service every year.
We can really run down the rabbit hole here, because in an ideal world, our battery powered computers can go the entire day without needing to be plugged in, and allow for charging overnight. Problem is, all those batteries need to come from somewhere too, and getting the material is usually also very costly.
Keep in mind that I don't practice this entirely either--I fall victim to shiny new things as much as the next guy.
No argument that mining is hurting us all. It's sapped the supply chain of premium GPUs, which has driven prices and production though the roof. A used RX 570 is over $200, when I can recall buying a brand new one for $110 2 years ago.