The WebPKI and DNSSEC run global PKIs because they routinely introduce untrusting strangers to each other. That's precisely not the SSH problem. Anything you do to bring up a new physical (or virtual) involves installing trust anchors on it; if you're in that position already, it actually harms security to have it trust a global public PKI.
The arguments for things like SSHFP and SSH-via-DNSSEC are really telling. It's like arguing that code signing certificates should be in the DNS PKI.