"The model of a key server with a transparency backend is based on the premise that a user is willing to trust the security of a centralized service, as long as it is subject to public scrutiny, and that can be easily discovered if it's compromised (so it is still possible to compromise the user's account, but the user will be able to know that as soon as possible)."So it's a central service where we can - eventually - find out if a key changes or is invalid, though not necessarily if anyone just breaks into the service. An attacker can still break in and monitor authentications to gather intel on users. Or they can automate an attack such that the keys are compromised and the attacker gets the access they want while you're asleep at 3am.
"a Monitor could notify the user whenever a key is rotated, and the user should be given a chance to revoke the key, and rotate it once again."
Now the user needs to see this e-mail about their key getting rotated, realize it's a compromise, issue a new rotation, and hope nothing bad happened in the meantime.
"making it so hard to hide a compromised key directory, it would almost require shutting down all internet access to the targeted victim."
This is completely within the scope of many different types of attacks used today, though often you only have to limit it to specific servers.
"The model envisioned in this document still relies on users being able to keep their account secure (against phishing for example) and their devices secure (against malware for example), and simply provides an easy-to-use key discovery mechanism on top of the user's existing account. For users with special security needs, we simply recommend they verify fingerprints manually, and we might make that easier in the future (with video or chat for example)."
So it really is just a PGP keyserver. For users who care about security, do something else to make yourselves more secure.