But they don't have all their eggs in one basket, lets look at this from a logical standpoint.
If HP does take over the rest of LC, then basically they would have swapped out their largest land, for...well their largest land.
The previous LC theme have two resturants, the existing Arabian section of shops, and a few snack carts scattered and counter service food.
If they convert they will have:
2 restaurants (3b's and currently Mythos), the Hogsmeade of shops as well as the Diagon Alley of shops, a few snack carts of fruits, butterbeer etc.. and maybe some quick food stands possibly.
Sounds like an even swap to me...NOW....if with the new section open, you can make 75% more profit than the previous section, would you do it? What if that number goes down to 20% 10 years from now, would you still do it? The answer is yes, because regardless you're making more money than you were previously, and that always feeds to the rest of the park as well as people buy merch from the other stores as well.