It works at small scale when you self-select for technical competency. It does not work at larger scale when that self selection is no longer possible.

My scale is that I ran an ISP for ~500 users before the network was disassembled last month. At that scale, you will encounter people that make mistakes with their email setups. When the people who make mistakes are customers which DMARC prevents delivery of emails, it is an issue as those are exactly the people for which I want to see the emails from.

I get more spam with valid SPF and DKIM via Google's own mail servers than DMARC blocks.

It says something when even gmail doesn't use DMARC as a signal that an email is valid, as gmail regularly blocks legitimate mailing list emails with completely valid signatures and non-spam content from a reputationaly sound IP.

The problem DMARC was supposed to solve (impersonation to reduce spam) isn't solved by DMARC.