It's still a net positive for candidates like Mamdani to achieve electoral victories. Even if you believe that a true socialist can never make it to a high enough office to establish a socialist government (which I agree is likely correct), making the attempt and achieving some reforms in the face of very public resistance from the bourgeoisie is great for class consciousness. It sends the message "hey, it actually is possible to improve your material conditions, and the rich really don't want it to happen." Give people a taste and they will want more, which is why the establishment is so terrified of Zohran Mamdani.