I think “slowing down the bots with proof of work” is essentially what captchas were, back when they weee easy for us and hard for computers.

But now, when I see a captcha, I hit back, press unsubscribe, and find a new vendor. The work is harder for me than it is for a computer and so I won’t do it.

Accordingly, we can see that proof of work is the opposite of a solution.