CT was introduced to detect misissued certificates, but it was never intended to be used as a timestamping system like this. The point of the SCT timestamp was to start the clock on the deadline for the log to publish the certificate, not for use in trust decisions. So when Chrome announced they were using SCT timestamps for trust decisions, my first question[1] was whether anyone was auditing CT logs to detect backdated timestamps. Since then, I have added timestamp auditing to my monitor and should be able to detect a CT log backdating an SCT timestamp.
The goal of CT was not to make staged distrust of a CA possible (that's just a happy coincidence). It was to make it possible to detect CAs misissuing certificates, which includes CAs back dating certificates.
