Not me being Grinch-y. It's a legitimate problem with the queue that has been seriously considered before (not a curtain, but removing the statue entirely).
The Oxford Corridor is where the line stops - for no functional reason. This causes the line to back up into the greenhouse. There are not as many people physically within the castle as it was designed to hold. They stop at the Bird and get stuck outside in the greenhouse.
The lockers are a similar choke point. You may note that you essentially never have to wait in the queue that's in the dungeons beyond the lockers, but rather rush right through it to the greenhouse. It's because the lockers are keeping people for far longer than they were designed to.
In a perfect world you'd approach the castle and go through the front gate (no extended forest queue) and walk right up to the building (with no Express/Disability line division, just a big open walkway) and then hit the lockers if needed and then begin the queue at the dungeons for the ride, or opt to go up the stairs/elevator to take the castle walking tour.
Guests would have time to consider the mirror of erseid (spell check?) and hear Neville be berated in the potions room.