Funny idea: a long form journalism site could intentionally and openly embed such a script, allowing readers to make "micropayments" of lent CPU time to the site while they read articles.
I'm pretty sure the factor five in your second point is in reality way worse than that, probably (wild guess) at least over 25x.
Mining is not only inefficient when implemented in JS (even with the web asm stuff, many clients have no support), or when it runs on crappy or old hardware, it also depends on what your electricity costs. Pro-miners put their farms in locations where electricity is super cheap, often in the middle of nowhere, perhaps near a hydro dam. Most clients are on a laptop or mobile in a city, where electricity is much more expensive, and their electric efficiency is made worse because they run it off battery.
Distributed crypto-mining for micropayments via embedded JS is such a huge waste, I'd probably have more problems if I knew the site was doing that, for the guilt of wasting that electricity while reading, just so the author can inefficiently receive a few fractions of a cent. I might even (gasp) rather watch a video ad instead.
They use Coinhive, but using it this way seems reasonable. Their strategy is to be open about it, to let users know their CPUs are being used to mine, with care not to drain mobile user's batteries. They only use it on long articles. I'd much rather encounter this than ads. It's much more convenient than a micropayment service, and doesn't preempt additional subscription income. I expect to see bespoke variations that don't trigger antivirus software. As long as mobile users' batteries aren't drained, and users' CPUs aren't overly stressed, it seems an ideal solution to monetize websites.
