Delaware Bay serves as one of the largest, if not the largest, nursery areas for sandbar sharks which make up the vast majority of little toothy critters you are referencing. As a whole sandbar populations have been lowered by almost 70% and the fishery closed with the exception of a handful of research permit holders primarily in NC. I only mention this as background to your idea that sandbars are depressing flounder stocks through competition. I don't think the work that has been done on feeding habits of these species would support competition as a major factor. As previously stated by someone above, sandbars feed primarily on other fishes (primarily croaker and menhaden) although they also like soft shell crabs (who doesn't?). I am not as familiar with summer flounder but adults are more piscivorous (fish feeders) but they still prey upon bottom dwelling invertebrates (worms/clam sipphons) at pretty high levels.
Fishing seems pretty good in DE this year based on the carcasses that I have seen at the marina and the guys I have been talking to.