I've read his response many times, including the day it was posted.
I find it to be thoughtful and thorough, although imbued with the passion that launched the experiment (and instructions) in the first place, which I surmise may be part of the critique you find unanswered?
May I ask which specific critiques you believe survive his response and impugn the study?
> the nearly 50 years between the experiment and its failure to replicate.
Obviously Zimbardo (and, it seems to me, a substantial chorus of reputable sociologists and prison reformers) have a different impression. I think we can safely characterize the response to the replication critique to be:
a) The attempts to replicate the study which have actually attempted to imbue the participants with the cultural and psychological accurements of prison life (including, sometimes, express expectations of toughness and control, which are real and documented aspects of standing orders at many prisons) have in fact successfully replicated the SPE - the NSW is the one Zimbardo cites in his response of course.
b) The popular "failure" in replication by the BBC, which is cited all over the internet every time this comes up (though I presume from your cautious wording, is not part of your critique) is very poorly constructed and has little in common with the parameters of the SPE.
c) Real-world prisons which have been studied at length, including some domestic maximum security or supermax prisons and also some detainment during the "global war on terror", have exhibited very similar outcomes to some of those observed in the SPE.
d) It is not possible in an academic context to perform the SPE anymore, as neither the instructions given nor the consent to prolonged stressful conditions are generally permitted in human experimentation anymore.
I find the SPE to be highly imperfect, but failure to replicate is not among the criticisms which seem to me to be justly leveled against it.