The analysis is fine, as far as it goes. The problem is that it ignores several intangible values.
One is the value associated with being able to send mail that recipients want to receive without recovering costs on a per-message basis. Some of us remember the days when long-distance phone calls were expensive, and if you needed to control costs there was no good way to keep in touch with distant friends or relatives or colleagues.
Another is the lost value of spam recipients' time, attention, and focus when they receive spam.
Another is the lost value of taking a reasonably reliable communications medium and making it much less reliable due to spam filters.
"Did you get the message I sent?"
"No, I didn't see it. It might be that I accidentally thought it was
spam, or maybe the spam filter got it."
The "obvious" solution is to find a way to charge the spammers. But this can never be sufficient by itself. The reason is that the various kinds of intangible value lost are priceless - they cannot be recovered with money. So it's "obvious" to me that we are going to continue to need other barriers besides monetary cost to stop spam, presuming of course that we want email to (again?) be a cost-effective and useful communications medium for everyone.