In all honesty, and this comes from experience, the average park guest who decides they won't come back due to over-crowding most likely would have never come back anyway, or would have waited five to ten years to return. I admit that I am totally new to the Theme Park world, so I still remember my train of thinking from being an average guest. Believe me, despite the unbearable conditions a day can induce, a general park guest is going to try to make the best of their day and get the best bang for their buck anyway. So the streets are small. There will be an entire park beyond Diagon Alley, including attractions like Despicable Me, Transformers (Oops, I mean... perma-haunt...), MIB, and Simpsons. Capacity may be a little short in the area, but like violarum stated, if you think it's that bad, go on the off-season. As a general park guest ten years ago, that is exactly what I used to do, and mind you, that was for the Disney Parks, which are always unbelievably busy, as any person here would know.