I think there's just a different measure of success. I think the socialist movement that built up the NHS in England with Bevan, the movement that built the Workers Coucils in France, the socialists that wons the 8 hour day globally, the Zapitistas, the PKK/YPG, and the rest of the socialist movements that built the modern welfare state could be considered successful.
I measure success more on the material conditions of the working class, rather than if the party has complete control over a country. Currently the democratic socialist movements have more control in the Democratic world, global South and global North, than the Leninists do.
The very second that China, Vietnam, Cuba, or Laos actually allows for free elections between multiple socialist factions, and not just the control of society by a party elite, that's the second I'll consider those leninists more successful than the Democratic Socialists.