People want to see the fireworks from in front of the castle. Get rid of projections, and people will still want to see the fireworks from Main Street/Hub.
The guests want the "postcard perfect" image/experience of seeing it from the hub.
Not all of them. And when there are perimeter launches, where do you get that "postcard perfect" shot from, standing in the hub or somewhere on Main Street? Not really.
There are also plenty of spots people used to watch Wishes! from: the beaches at GF, Poly, and the Contemporary, the top of the Contemporary, outside the 4th floor of the Contemporary, on boats in Bay Lake, on boats in Seven Seas Lagoon... Unless you're going to eliminate repeat guests, those are extremely viable options for someone who has seen a fireworks show at MK (without projections) at least once before. Disney created the entire problem by thinking projections on the castle were a good idea. Cramming everyone into that space creates immense pressure before the show and after it is done. Eliminate the problem and guess what happens? You start relieving the pressure on the area.