Oddbean new post about login | logout Why do you think so? Again, spammers are absolutely willing to pay. It's by paying that they have become so prominent on the web. The fact that the cost is "verifiable" doesn't make much of a difference.
Just look at Twitter. I would say 99% of bots haven't the blue check. It's a matter of economic margin. There are spamming methods that are economical at 5¢ that aren't at 10$. And, this PoW stuff that I am talking about, is to be used to help a new user bootstrap his own web of not spam.