Your logic is truly dizzying...and that isn't a good thing. The investment has already been made....that's why there will be depreciation. Having depreciation means you spent money in the past, it doesn't say anything about whether you will or should spend money in the future. If Uni spent $400 million building Potter-land, and believes that what it built will last 20 years, there will be $20 million per year in depreciation expense whether Uni spends a lot in the future, or a little. In order to show profits, that means that the capital expenditure must bring in $20 million/per year more in income than would have been there without it.
The "giant depreciation bubble" you speak of is merely a fixed cost. Like all other fixed costs, if the fixed cost is higher than the income it produces, that would be bad. This applies to Potter, Avatar, New Fantasyland, and everything else. If what you are suggesting is that you don't believe Potter will continue to bring in enough income to cover the investment, that's fine. Obviously, the majority view around here is that it will. Indeed, it probably already has covered a huge part of its capital costs given the attendance bump and food and merch sales.