I’m not sure this isn’t overthinking it a bit.
there is a “fixed cost” to hold the party. Once costs are met by cash paying customers, there is a small (likely tiny) incremental cost for each guest. But in general, once costs are hit, margins expand pretty quick on these events. Just don’t know what the break even point is.
I think the ‘sold out‘ broadcasts is intended to whip up the scarcity (fear of missing out) in shoppers. Drive a decision towards sooner and ‘yes’.
‘fear of missing out‘ on a big, expensive trip is a powerful motivator…I’m guessing Disney is leaning hard on that tactic.