I appreciate the additional insights, but the premise I'm pushing back on is whenever a SSH key is read, then the user account is by necessity compromised in order to do so. Given that level of a breach, there are myriad ways for an attacker to escalate privilege and exploit their access without worrying about a password on the SSH key. Namely, at that point, cracking the password on the SSH key is a tractable problem.
> They may be using directory traversal from something else to read a key without even necessarily being on the system.
At least on linux - to read the directory containing a SSH key requires the ability to also write to that directory, as the user. Therefore you can also write to '.bashrc' and all sorts of other places. I suspect Windows might have a larger attack surface, but nonetheless, a directory traversal that is able to read and write is also able to install a keylogger.
> They may be on a server somewhere where someone left a key they shouldn't have
Private key should never be transmitted over a network boundary. SSH key passwords can be bruteforced as well. Having a password on the SSH key, when the SSH key is somewhere it really should never have been, is closing the barn door after the horses have left.
> The attacker may have gotten enough of a secret to compromise some other secret store where the key is being held.
Again, getting access to the secret is enough to also have write access and be able to install a key logger. A password on the SSH key still does not help.
> They're probably on a system with user-level access only and that may not be enough to "just" install a keylogger
If a person has enough access to read a SSH key, they can also install a key logger for at least that user account. They are equivalent levels of compromise, a user account having its SSH key read is already compromised.
edit: addendum: There are certainly attacks that can only read the contents of a system, with root that can read the full system. It's just odd to think about, since at that rate the SSH keys being on a prod system is already such a big no-no. SSH keys really need to live exactly just on the personal devices of the people who own those keys - EG: it should never be the case that say a SQL injection attack that gains root level read permission over everything on a filesystem can then ever read SSH keys - cause those keys should never be on the remote system to begin with. Putting a password on private keys that are then copied to servers _is_ security theater; the keys ought to never be copied to a remote server to begin with.