_Possible_ but NOT certain. It depends on the uncertain outcomes of multiple struggles.
That is not remotely related to anything that could be called "Optimism." And I suspect Russell uses "optimism" in the technical sense in which it was used in reference to 18th-c theodicies. "Best of all possible worlds" etc.
Carrol
P.S. I think very highly of Russell; I learned a great deal back in the '40s & '50s reading his works. But he was no expert on Marx or Marx's thought. And there was no reason he should have been.