That's an easy thing to say after the fact indeed but yes. In fact after such a disastrous backdoor I wouldn't be surprised if OpenSSH moved all code calling external libraries to unprivileged processes to make sure such an attack can never have such a dramatic effect (an auth bypass would still likely be possible, but that's still way better than a root RCE…).

At this point “All libraries could be malicious” is a threat model that must be considered for something as security critical as OpenSSH.