If you “ran” the most worker-centric paradise, you’d be the elected CEO of a co-op where the workers had democratic control of the means of production. In such a place, there’d of course be no need for a union because the workers would have the say in what happens already.

Now your strawman benevolent dictatorship could never exist since you would still be denying workers control over their workplace. Thus they may form a union to fight for that control.