Well, the consideration fails unless the possibility of a common origin is excluded. Just because we find life nearby doesn't mean that the great filter is not already in our past. On Mars specifically, it would be near enough to allow for the possibility that life has somehow migrated from Earth to Mars, or vice versa, or to both from some other source. Finding life far enough away that intelligently developed spaceflight would be the only way for it to migrate would indeed be a depressing thought, but the possibility of migration must be ruled out first if we find life nearby.