This is spot on. They have literally no clue how many multi-night ticketholders will appear on a given night, and many of them have express. This arguably puts them in a worse position than Disneyland pre-park reservations, as attendance of express multi-nighters can dilute the efficacy of their premium product. The only levers they have to pull are declaring a night "sold out" (thus discouraging multi-night ticketholders from attending) and exaggerating wait times, which has other unfortunate downstream effects. Neither are sustainable.
I hate to say it, because I hate park reservations... but if they don't make other drastic changes to the multi-night tickets, I think they're going to need a park reservation mechanism. Otherwise, you gotta either sell way fewer of them, double the price, or remove the express element. No solution is going to keep everyone happy.