Well, perhaps people would make supper their last activity of the day at DHS, eating at 5 or 6, then heading out rather than stake out their seat for Fantasmic, in which case it would have little effect on dining. I obviously don't know what everyone else's habit is, but my wife and I ate supper after we were pretty well done at the park, and then headed towards Fantasmic after we were done.
Or, perhaps many people would leave the park and go have supper somewhere else. If they ate on property, then there would be a negative impact on dining at DHS, but they would still be eating at a restaraunt somewhere at WDW, so in the final analysis, the Mouse is still getting your dining dollars, whether you eat at Brown Derby or Tony's Town Square, or the restaraunt back at your resort. I'm not sure how important it is to ensure that a particular park makes a certain dollar amount vs the overall bottom line of the entire resort complex, so I don't know how concerned they would be at the loss of dining dollars at DHS if they thought the majority of those dollars were being spent at another WDW restaraunt.