You mean just like ads do? Of course, in a perfect world we wouldn't have either, but I just don't see clear distinction between mining in browser or ads or other useless/annoying things (from the user perspective).
One of those disincentivizes waste (howewer weakly). The other incentivizes it.
> playing computer games, watching mindless YouTube videos, scrolling Facebook, leaving devices on when not used, lighting places 24/7 where nobody goes at night anyway etc.?
We do. But all those examples are the case where energy use gives us something (fun, convenience) and where the waste is bounded (there're only so many devices I can leave powered, and also I have to pay for the power they use). Crypto mining literally pays you for wasting power, so the more electricity you can burn, the more money you get. And the more people join in, the more you have to burn to keep up. So there's almost no ceiling to how much personally you get to waste, and a strong incentive to waste as much as you can.
And what's all of that for? To replace a system that works fine and does not have a greed-powered feedback loop of waste? That's just dumb.
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[0] - Currently. Hopefully they'll always stay that way, but there's an off chance this becomes so popular that all new computers will come equipped with additional hardware and drivers to mine crypto for micropayments.
Well, you (or somebody else) still has to pay for power consumption used for mining, so it just converts money you pay for energy bill to micropayments (though in very unfavorable rate as of yet). Don't get me wrong, I don't advocate such shady behavior in any way, just wanted to point out that this "wasted power" point is a bit of a stretch.