When it comes to the french revolution the answer is easy: This was not a workers revolution - it was a bourgeois revolution. It means that feudal lords were replaced with capitalists. As you said, the power went from one small group to another small group, but it still changed an economic system - feudalism to capitalism. The russian revolution is still the better example that should be studied. It abolished feudalism and got a step further to a form of socialist experiment.

I guess there will always be bad people that are hungry for power and have no morals, as you said - no matter the system. Corruption will always be a thing, but it is important to have a system that sanctions it properly and that allows society to have a clear moral stance on such behaviour.