I don't think it was meant that the timeshare resort itself fails if the resort company doesn't move on to a new one. I think the issue there is that once the resort is sold out, the timeshare builder stops making any profit. So the maintenance fees may pay for upkeep, housekeeping, etc., that's really all it does.
I'm guessing the way to think of it is like a home builder, probably if the builder kept some degree of control over the HOA and the common areas after completion. Once the neighborhood is done being built and all the homes are sold, the builder could continue to get some money by maintaining the common area, etc., but it really only makes its money when it sells the houses.
While it's not a completely apt comparison since Disney is able to make money off of DVC properties in other ways: buying back points through Right of First Refusal and foreclosure, interest on the DVC lending, and the big one the most DVC guests are spending lots of money in the parks and restaurants, etc., Disney's making its real money off the sale of the DVC properties (strictly as it relates to their timeshare division). If they stop building the resorts, then they are limited to making money off of those other areas. So while DVC makes a ton of money for them up front, hotels make a return on the investment of building them over a longer period of time since they continue to make money.
Now Disney also has the advantage of contracts expiring, so that, I believe, will allow them to then either tear down and rebuild, or just gut and refurbish, then re-sell the entire resort's allotment of points again. While the expirations are advantageous to buyers in that they are not stuck with a property that their family has to deal with in perpetuity (not everyone wants to inherit a timeshare), this also allows Disney to deal with the finite amount of land it has in the area and eventually recycle the properties and make money all over again.