There are (at least) two reasons why I think the doomsday scenario doesn't happen.
First, it's just not the case that everyone can plan at 11 months. Some people can, sure. But, not everyone can or will. Riviera has a decent room mix. There are 210 studios in total between dedicated and lockoff, with about 280 larger rooms. If you absolutely need a Tower Studio (or a Standard View) then maybe things are hard much of the time. And yes, it's inconceivable to most of us that someone would not be online right at 8AM, 11 months to the day before a vacation that we might or might not take. But most people just don't think about this the way the average DISer does.
Second, I suspect it will take a while for even an appreciable minority of the resort to be in the hands of resale buyers. We see things like "people hold contracts for ten years" but as I understand it, that's only the people who are selling. Lots of people don't. I have yet to see anyone do the work to really figure this out*, and it would be hard to use other systems as an estimator because the economics are different for most of them.
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*: I'm not even 100% sure that the public deeds give enough information to do this, though I think if you had access to all of them you could probably figure it out. But you can't just count the total resales, because some contracts are presumably "resold" more than once.