I think the horn is the best way to indicate impending stall. In many (perhaps most) WWII aircraft, there were no stall horns; and aircraft differred on how much indication each gave of impending stall. However, other than a horn, the indication would be a feeling of buffetting, and that is much harder to convey just with screen shake or a subtle sound -- such effects are not as noticable as having your body feel the buffetting. In computer simulations, you have to substitute one sensory modality for another, and it is important for the substituted modality to be as easy to notice as what it is substituded for.
For me, having flown games without the horn but only with screen shake and buffetting noise (like WWIIOL), I much prefer the horn as a warning of impending stall.