At last count, I have had 49 articles published in Dragon and 8 adventures published in Dungeon. Of course, one of the Dragon "articles" was an adventure, so maybe I should count it as 48/9. In any case, the vast majority of these were in the 2E days.
I had about three submission ideas to Dungeon shot down before I finally got "Challenge of Champions" through (issue 58), and nearly a dozen Dragon articles shot down before they finally went with "The False Undead" (issue 198). I would recommend to Magic Slim that it's quite likely he's taking the wrong message from his situation. You never know, the next idea you submit could be the one that makes it through, and once you've been published it only gets easier from there!
Johnathan