It’s not meant to be a full protection mechanism. It’s meant to make something that’s near infinity cheap (like sending emails), have a small proof of cost.

There then can be a market value where a spam message may have a $0.005 return based on clicks and people you steal money from. If clients opt in to requiring above that for event pow, you kill the mass spam use case economically.

Does that prevent all spam. No. Does it prevent targeted phishing. No. Is a single pow min value the same for everyone or every case. No. If the spam is at cost and just noise, again no protection as sender isn’t trying to make money.

And you ignore pow derivatives. A single high pow event could be ranked down to a max score. A sum over time shows value over time. And interactions with other high pow can have a similar to page rank to give value to others you/they interact with.