I don't think the issue is that 'good' dictators are so rare (one in a million), but that they are rare enough that two back to back is extremely rare and three back to back is beyond hope, even if each dictator tries their best to train up their replacement. As such the real test of these systems is how they fair after the original dictator is gone. Singapore is currently in the middle of their test and the grade won't be seen for a while.