The physical reason not to prop up the democrats is that the democrats are leading the US to the end of the world, just like the republicans, and legitimizing their rule makes leftist organizing more difficult.
Drag already asked how voting for the democrats legitimises them, and you didn’t answer except for a symbolic reason. Drag wants a practical reason. Do you understand drag’s critique? You gave a “practical” reason made out of symbolic reasons. Drag wants a practical reason made out of practical reasons.
Legitimacy comes from the consent of the governed.
If the governed stop voting, the legitimacy goes away.
I already answered. Voting Dem perpetuates Dem rule, if Leftists are pushed to vote Dem and see it as activism then the Dems take advantage of that and portray voting Dem as activism. Feeding the electoral system by voting for either dictatorial establishment party maintains their rule and legitimizes the system.