In your response you make three bad arguments:
The first is based on a claim of universal morality even across temporal periods. You make a claim "To subjugate another entirely to your own will is never moral, never ethical, it is a fundamental debasement of that person's humanity." that is based on western conceptions of freedom that do not translate across all borders and certainty not along temporal ones. This is quite a claim and most modern thinkers, even humanitarians still argue that historical norms are very different than modern ones and it is incredibly difficult to judge the moral actions of past states. In some societies giving up ones "will" to the greater good has not merely been the accepted but has been the highest moral goal. Would not forced conscription fulfill your claim, certainly the type of soldier in antiquity at least? Yet the levied peasant classes of Greece served and attained great personal honor to die for their city and for their city elites. Heredity systems also remove will and choice by forcing heirs into responsibilities they have no choice over, yet we in the West have embraced hereditary systems of inheritance gladly and see attempts to remove it as attacking our freedoms (which is ironic because inheritance removes our freedoms). My point is that your moral definition of slavery cannot apply across temporal gaps let alone borders, you are imposing your will on other societies and enslaving others to your own moral code. There is no evidence that other societies or peoples uphold those views nor that they should be considered universal.
The second bad argument deals with your claim "The fact that every now and then a slave is freed or achieves wealth is not a validation of slavery." Yet the cases I looked at were not outliers but rather common occurrences in eastern societies. Slavery is a function of a society and all functions of society can ebb. Slavery can ebb towards the evil and ebb towards the good. If you deny this you once again make bombastic moral claims which weaken your argument. Slavery can be made good and societies often try to make slaves, the poor working class lives better and can achieve even the highest of prosperity.
Thirdly you made a claim about "every dominant society and show that there was a small 'elite' class and a large 'working class'." Here you are simply wrong there have been a great number of societies of equals. These cities of farmers or clergy or thinkers or peasants over time have made both regional powers and even small nations (Tangier, The Amish, Singapore, Sweden, etc) but since your thesis rests on only looking at supposed nations which you can't even name you are guilty of selection bias and your argument is unfounded.
In the end it is not that I am nit-picking, it is that you are picking ONLY what supports your argument and ignoring all else. I am asking you to consider that your cases are wrong and you are telling me that you can go and pick different cases. This does not make your argument stronger, it weakens it by showing your inability to defend your case selection. You have made sweeping generalities about morality, failed to recognize the prosperity associated with some kinds of slavery and absolutely misunderstood your own chosen cases.