Perhaps the key here is "for decades" and "reputable ip". I do run my email from a dedicated box - but I host with hetzner. Their ip blocks might suffer from "bad neighbour" effect.

But how is such an effect not an issue? It makes it hard to "elevate" a box at home to a proper mail server, and makes it hard for new generations to host their own mail (until an ip6-only world (and probably also then) all new users will need to recycle old ips).

As for dmarc, I'm still not entirely convinced it's a great solution to smtp/spam issues. Dkim is already annoying enough if you occasionally want to send mail through/from a different smtp - like Gmail if you don't run your own webmail.