IMHO the way they are handling it is brilliant. They could let day guests on Gringotts, half would get good rides and decide they don't need to come back next year to see it again; the other half would wait hours just to get dumped or evaced, and spread bad word of mouth.
This way they get all the money from mech & food sales, and all these people will want to come back next vacation for the ride. Profit!