#1 More room

WWE has a habit of wanting to do dream matches at Super ShowDown, and while they sometimes turn out great, they still take spots away from the younger talent. That's time that could have been used to help establish a new Superstar, further a storyline, or even create one, but they instead want to spend it on these dream matches.
Again, there is absolutely nothing wrong with dream matches and it can sometimes be the best thing on the show, but that doesn't take away from the fact that it hurts the product. Beyond that, it sets a precedent that alludes to WWE being more worried about what will do well now and not of what is good for the future of the product.
In the end, the product would benefit so much more from mid-card Champions defending their title or a women's match than they ever would with something involving the legends. That's why WWE needs to choose to invest in the future of the product and use Super ShowDown as an opportunity to build a Superstar.