

Social democracy isn’t really an alternative to capitalism, it’s more of a progressive modification of capitalism. Under social democracy, the factory would still be owned by a capitalist and operated for their profit, but the government would use its authority to regulate the factory, tax the owner’s profits (to help fund public services), protect workers, etc.
It really depends on how you define “successful.” If your measure of success is based on how closely these societies resemble Western, liberal, capitalist societies, then, yeah, you’re probably not going to see a whole lot of “success,” but that’s not what these revolutionary movements were trying to achieve. I would say that first and foremost what essentially every communist movement was striving for was just autonomy and independence, and many have been successful in that regard. Vietnam is an independent nation, instead of a French colony. China, similarly, is no longer under the thumb of the British. You may not like what these nations do with their autonomy, but that is what they were striving for and they have achieved it.