There are two situations in which timeshares are "worthless". The first (and the most important) is if it is "under water"--defined as annual fees that exceed comparable rental rates.
This can happen in a weeks-based timeshare in a highly seasonal destination. In these resorts, each owned week pays ~1/50th of the ongoing costs (assuming two weeks held back for maintenance, etc.). But, the rental "values" of in-season weeks are signficantly higher than the rental values of off-season weeks. If the resort is very highly seasonal it is possible for the offseason owners to be under water, even though the high-season owners still get very good value.
When this happens, a seasonal weeks-based resort can enter a "death spiral" in which off-season owners stop paying and allow their units to be foreclosed. Those weeks can't easily be sold, and while the resort will try to rent them, the rental proceeds are not high enough to offset the lost fees. The lost income is then spread over the remaining owners. This pushes more weeks underwater, and those that were already underwater go farther under. More owners default, rinse, repeat. Eventually the resort ceases operations and is sold off, with the proceeds going to remaining owners.
There are a bunch of ways to deal with this. For example, several of the northern Michigan resorts originally sold their weeks as a bundle of two or more with a mix of seasons. So instead of some owners having a great deal and others a terrible one, every owner on average has a "good" deal.
However, the easiest way to do it is to sell as points rather than weeks. In a points-based resort, the weeks that are worth less in rental value also require less points to book. The weeks worth more in rental value require more points to book. If the points allocation is done with at least some care, then if the resort has any value at all, everyone has a more or less equal share in that value.
That's usually good, but it can be very bad. In this model, either the entire resort has value, or the entire resort is under water. If that happens, you get the same death spiral, but it happens much more quickly. The good news is that such underwater resorts are uncommon. This generally only happens to resorts that are in very "regional" destinations without much broader appeal, and that suffer significantly in comparison to any other options in the area. The latter happens because the owners want to keep fees low, so they don't invest much in refurbishments, etc. That's a strategy that can backfire if the resort falls too far behind the competition.
I suspect Vero will never get to that point. I took a quick look at other Hutchinson Island spots that appear to be of comparable quality (Expedia lists Vero as 3.5 stars, so I'm looking at "branded" 3 star or general 4 star properties), the all-in weekly rate is about $2-3K for a standard room w/not much of a view for early June. A standard-view "Inn room" that time of year is 104 points for a week, unsubsidized dues are $12.85, for a total cost of about $1,340. There's still a healthy gap between those, which is at least partly why Vero is still worth something on the secondary market.
Vero isn't the ideal location for FL Atlantic Coast---its a little too far north for snowbirds---but Hutchinson Island is still a decent draw. It's also unlikely that Vero will fall significantly behind the competition in terms of upkeep.
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There is another reason why a timeshare might be "worthless" (meaning: it has to be given away rather than sold on the secondary market)---there isn't enough organic demand in the secondary market to meet the supply of willing sellers. That can happen even if the timeshare itself "has value" from the rental-rate perspective, because timeshare is something that is sold, not bought. Very few people (outside of the DVC section of Disboards, TUG, etc.) wake up and decide they are going to go looking to buy a timeshare. If there isn't enough traffic in the market, it doesn't matter how much "value" the week holds--you aren't going to get a great price for it.
This is more likely to happen to resorts that are unbranded. Unbranded resorts are a problem because no one is going to go looking for one. Everyone knows Disney. The name automatically carries some sense of value and desirability. But "Generic Beach Resort" that isn't affiliated with one of the name-brand hoteliers? That's a much tougher sell---even if it is a perfectly fine resort, and the fees are less than area rental rates.
Even if this happens to a timeshare you own, it's not the worst thing in the world, because there will still be plenty of value in using it. Renting the same thing would cost more, and if you still want to go, great! If you no longer want to go, you can probably find someone among your social circle who would be wililng to take it off your hands--again, because it has value.
Even so, this is not likely to happen to Disney, because the fan base is large, passionate, and well connected. I can't think of another timeshare system with a dozen or so well-known brokers handling resales. That's not because they are worthless: Hilton, Marriott, Hyatt, etc. all have real value on the secondary market. But the Disney market is "just different," and I don't see that changing anytime soon.