> I can imagine it being ten or hundred times more
I don't think I average even 2 captchas a day being terminally online, so 10 across every soul in the world sounds way too much for me. (we're ignoring bots it's meant to deter?)
> it's hard to see that such a modest energy cost would dissuade any attacks.
It's not against targeted attacks, but scrapping.
And not about energy cost, but available compute power -- it requires scrapper to use browser with JS (or time commitment to reimplement PoW outside of JS), limits their request rate by CPU core count.