The absence of those future people in the world means more resources for other future people. If the population is at equilibrium, the effect on getting rid of the legacies of the people you killed is exactly balanced out by the increase in the legacies of other people who now have the resources to exist and are using them instead.
(If the population is growing, or if the population is in a steady state that is not an equilibrium because when it is pushed to one side it stays that way, then killing someone can take away legacies as you suggest. But you can’t then conclude that this is bad unless you also want to accept that it is good to add people, which leads you to the repugnant conclusion.)
The absence of those future people in the world means more resources for other future people. If the population is at equilibrium, the effect on getting rid of the legacies of the people you killed is exactly balanced out by the increase in the legacies of other people who now have the resources to exist and are using them instead.
(If the population is growing, or if the population is in a steady state that is not an equilibrium because when it is pushed to one side it stays that way, then killing someone can take away legacies as you suggest. But you can’t then conclude that this is bad unless you also want to accept that it is good to add people, which leads you to the repugnant conclusion.)