The point of SPF/DKIM/DMARC is to bind emails to domains, so no more spoofing. It is naive to expect authentication alone can reduce spams.
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DMARC is nice though. It won't stop spam. It won't stop spoofing. But you will know that someone somewhere is spamming people using your domain name. How awesome. :)
Of course, even with hard fail spf and dmarc, I still see some bounces from spam where some server accepted the mail to deliver it elsewhere and the next server denies it, so the first server sends me a bounce.