I really like the idea of this as a captcha alternative, especially with the "train our neural nets" approach google has been taking recently with them.
How would coinhive work as a captcha alternative? There is no human involvement in mining, is there?
Not saying it would be a good idea but something like "Please wait until your CPU have done enough amount of work before submitting this comment"
rate-limiting is not what captcha does, that's for DDoS protection and even in that scenario I feel HashCash and crypto mining is a very terrible idea.
Hence "captcha alternative" and not "alternative captcha".
Wouldn't "captcha alternative" imply that the said alternative is a replacement for captcha and has similar functionality?
Not necessarily. If the goal is to reduce bot activity, captchas and hashcash are two ways to do that. Captchas are hard for bots to pass, whereas hashcash wastes a fixed amount of CPU time. The result is that robot operators have to invest more effort to abuse your service at scale than they're willing to (hopefully). In this sense, hashcash is a captcha alternative—it's not a captcha, but it helps with the same problem. An alternative captcha might be using something other than reCaptcha. At least, that's how I'd interpret it.
The unwashed masses can't understand the difference between different coins, so you might as well call everything Bitcoin.
Most of these crypto currency exchange are derived from bitcoin's codebase (or at least from bitcoin's white paper) so from that perspective I don't consider calling these other crypto's as "bitcoin" to be incorrect, just not specific. I could imagine that maybe could use the distinction of capital Bitcoin to refer to the specific blockchain while lowercase bitcoin to refer to any Bitcoin forks.