This is absolutely what Disney believes will happen (hence why they don't seem to care about it). I will say I have NO knowledge or experience in tourism or travel patterns, so I'd assume they know what they are seeing and why there is good reason to believe your scenario is correct. I'd also assume the idea there is NO worry is probably incorrect (not that you said that here) just because they are making some assumptions, and there is a bit of an inherent risk in that.
So, my question would be, is there a point where Disney would care, and if so, what is it. If in your scenario, Disney only went to 102 guests, while every other year they were increasing by 10%, would that set off alarm bells? Would that one year matter, or did they build into their planning the idea that they would see a single year drop, but it would stabilize? Like I said, no real knowledge, but I do find this really interesting.