It's the only absolutely reliable way of getting rid of bad leaders.
This might not be a good enough reason to oppose longevity tech, but I don't think it's easily disposed of.
Death gets rid of good leaders and bad leaders equally. Other methods are more likely to get rid of bad leaders. As such, death of leaders would result in an increase in the density of bad leaders.
It's the only absolutely reliable way of getting rid of bad leaders.
This might not be a good enough reason to oppose longevity tech, but I don't think it's easily disposed of.