> chef/puppet/etc ... put just your pubkeys somewhere (S3, eg) ...
Yea, cool, and now you have a second authentication system that you have to manually administer to onboard and offboard people. And you have permanent long-lived single-factor credentials on a bunch of (possibly unmanaged) endpoints, that are easy to exfiltrate, and are a common target for attackers if a laptop is lost/stolen/compromised.
You haven't addressed host name reuse or host rekeying which, again, are real operational problems.
And somehow running and administering chef/puppet/etc or this S3 bucket is easier than running a simple certificate signing service connected to SSO that requires basically zero maintenance once it's setup? Argumentative, at best.
> Next problem is that openssh-style certs do not support cert chains ... recover from compromise.
Since only hopeless organizations don't have config management, we can solve this there. SSH PKI is simple and doesn't have intermediates. But you can configure multiple roots. To rotate or revoke a compromised private key you'd simply deploy a new root using cert management and issue new certificates. In a non-emergency situation you can slowly roll certificates by having two trusted roots for a transitional period while everyone gets new certs.
> You have to secure the CA!
You have to secure your configuration management that's deploying public keys everywhere and your authorized keys files on every host! Pubkey distribution has a bigger attack surface area and the same exact compromise consequences as a CA. This is not even a contest.
> You have to properly authorize signing requests!
You have to authorize requests to deploy public keys!
There are tools for this. What's the hard part here? Verifying an OIDC identity token? Mapping a token subject to an SSH principal?
Worst case, you could keep whatever crappy manual process you have for authorizing public key distribution and issue a certificate instead.
> These things fall into the realm of hard.
Ok, I guess? This is all relative. It's easier than pubkey authentication at any non-trivial scale. There's a difference between something being hard vs. being unfamiliar.