You want a wise, just dictator for life, and maybe you luck out and get one who reigns beneficently for a few decades, but who's to say their successor will be as good, or the next one, and on and on until you inevitably get a Stalin.

Put another way: if the leader dies, and heir A spends all their effort on ruling effectively, and heir B spends all their effort on beating heir A to the throne, how can heir A succeed? You'd need metrics for being a good ruler which are immune to Goodhart's Law.

Given that—over time—power inevitably centralizes and inevitably is abused, those tendencies must be counterbalanced; or else power must be regularly reset by more crude means.