My theory is that only Suga and Jimin were dead. Indeed, we can't see the others die in I Need U and even if we didn't know if Jungkook was dead or not, the yoonkook part makes me think that he's not. Jungkook is pretty depressed looking at the horizon, but Suga come and pat him to give him the strength to go on. And I don't think that everyone's depression in I Need You was because of a girl, but because of Suga or Jimin (or both) who comitted suicide.