Where Has All the Viagra Spam Gone?

Same for Nigerians seeking to transfer millions of dollars to me (if I give them my bank account number).
I haven’t gotten one of these in a year, after often getting several a day. I assume that the spammers realized that the return per period of time — the price of the activity — was less than its marginal cost: the opportunity cost of their time. They have shut down the business and moved to other activities that might yield higher returns.
Economists proposed taxes on mass emailing, but the market seems to have solved the problem. Spam still exists, though; the latest is the email saying I’m the nearest surviving relative of XX [pick some first name] Hamermesh, and if I would just send my bank details to a particular large South African bank, they will wire my inheritance.