Ummm... DKIM is NOT a one-liner in DNS. It requires actually setting up, well, DKIM. The DNS part is simple, yes, but the DKIM isn't, and equating it to a one-liner in DNS is just simply not true.
Importantly you can also verify the correctness of the configuration, which isn’t the case for all anti-spam and anti-spoofing measures.
I have had to revisit these a few times for companies that change their setup standards and then dont tell their customers (same has happened to SPF but that is much easier to audit/fix).