Why do we believe a stage show is coming? When was the last time Disney opened a major show designed for a dedicated theater? Have they done one since Nemo, which they recently chopped up because they don’t want guests in theaters?
Monsters isn’t a musical. It’s also an IP without adult human characters. Instead, the cast is full of creatures with inhuman body dimensions. Unless you’re just going to use the existing Mike and Sully outfits, which would be awful, you’re probably going to be relying on screens. That would mean something like Muppets, a type of attraction Disney clearly sees as dead and undesirable, or something like MILF, which doesn’t fit in that theater space at all.
Disney stuck that line in the release because, outside of the coaster, this is a fairly cheap overlay and they don’t want to remove or significantly alter the theater structure but having an empty venue of that size (especially after killing Muppets) looks bad.