I think a lot of this comes back to something that's very normal in late-stage capitalism: there's no such thing as "enough." they can never make "enough" money or have "enough" crowd-driving IPs or "enough" ways to further monetize their product. other factors, like budget and licensing availability, may limit what they can put together in a single event, but I don't think (and could be wrong!) they consider diminishing returns. at least, not very much.
edit: I'll add that it's not just how many people will show up to an event, it's what kind of people will show up based on the popularity or reach of certain properties. they want as favorable a mix as possible, which means more people who are willing and able to open their wallets for upcharge experiences, alcohol and merch.
It becomes a hard numbers game then because for example Demon Slayer film made 500 million dollars and most theaters like Anime films because those fans buy soda/popcorn.
I know they don't want to spend money but if they made a Demon Slayer cart with items for the I{, they'd sell a shot ton more then they did with the very limited Nope Merch they made.
It's why I'd love to see the process because from how I see it they try to have like One Big IP a year, Ghostbusters, Stranger Things, Last of Us.....etc and then fill the rest of the event with mazes that high light the IPs they want like Black Phone, Nope etc last year. They throw a bone to creative people and let them make a maze or two that are original because they don't have to pay an royalties and the Hotel from last year only happened because another IP is rumored to fall out and they needed something so made an Original IP
Because if USH wanted all nights to sell out and make more on merch they'd have be trying to hit more Demo's
Demon Slayer - Anime
Last of Us - Video games/TV (doesn't matter if its not based on the show people from the show are going to want to see this maze)
Scream - Biggest horror film of the year
Chucky/Megan - Controversial, I'm sure but I feel like its a waste not to just have both in one maze.
Resident Evil 4 - Sold very well and RE is a very hot IP
Stranger Things - Last season was huge and had a perfect monster for the event
But......that's not the event we are getting. Which is what leads me to think for the most part its about selling there own I{ and making the event cost as cheap as possible because most fans will show up whether or not their favorite IP is there or not.