Using authoritarianism is a fast track to ensuring that new generations of people see leftism as the enemy, if they don't already feel that way.

Taking away agency of people doesn't create lasting social systems; the masses aren't inspired to uphold the values by fear of consequence alone. People en mass will never accept being told what to do if they feel it takes away their agency. If anything you only breed resistance and contempt for your system of choice.

Who would you even trust to have such power? How could they be expected to uphold their values? How do you know that authoritarian power can't/won't be corrupted? Are you going to allow the murder of others if it cast as 'just' in the eyes of that power?

Authoritarianism had its hour, and in that time blighted all it touched. If it weren't for authoritarian leftists muddying the waters of the term 'Communist', Id imagine we'd have seen a lot more societies adopting those ideas instead of equating 'communism' with authoritarianism and rejecting it outright