Sam2: Where do we find such a concept as a “national Kaddish?” And one cannot be certain that the relatives will say it on the yahrzeit — because after all, the atheist “sages” of the state of Israel have “paskened” that Kaddish is to be recited on 4 Iyar.
As to why I assume that the Zechus is any different on a Yahrtzeit than any other day. I assume the Chachmei HaTorah who established these minhagim had good reasons for doing so. If they determined that Kaddish Yasom should be said on the yahrzeit of the niftar, then that’s the time that Kaddish will have an iluy le’nishmasam, not any old Monteg or Donnershteg. I did point out that I have no proof that it harms or distresses the neshama of the niftar, but it surely cannot have the especial segula that it has on the yahrzeit.