Therefore, RPC-to-publisher is at least as capable as SPF-like
There are a lot of small domains out there. I believe that we can get
many of them to take 5 minutes to add a DNS record. But getting them to
install new software, and configure a new web service, is way past the
ability of many of them.
Then they'd need to rely on the services of a third party (which is what
might happen with reputation services anyway), or not benefit. So this
isn't, on the face of it, a meaningful objection.