What is stopping them...well, the only thing I can think of is financial gain.
Let me explain. First, I do not see a halt to building new resorts for the near future. Discussions on here have even expressed an idea that there are at least four more in discussion. These could be on WDW property or in California, Tokyo, Hong Kong, Paris, etc. If
DVC lets their current properties slide into disrepair, they would be hard pressed to sell any new ones. Thus, it is in their near future interest to maintain their resorts.
Secondly, Disney owns part of each resort and rents the rooms to cash paying guests. To do this, they will need to maintain the exterior and amenities anyway. Plus, there are no designated rooms for DVC owners and those staying on cash. It is in Disney's current interest to maintain the resorts and bring in cash gusets at a very high rate.
Third, at the end of our contracts, Disney can either resell the contracts or rebuild on the site for somethign new (another DVC site, possibly). If they maintain the resorts, they could be in great shape in 50 years, thus can be resold at a premium (either back to the current owners at a discount or new ones). Thus, it is in DVC's long term interest to keep the resorts nice.
Lastly, we as owners foot most of the maintenance bill! Disney has no reason not to keep the places in good condition, since the bulk of the cost probably comes from you and me and not the company. They would be happy to handle this for us, especially since they get a management fee for doing so. Also, we would sue the crap out of them if they took our maintenance dues and the places fell apart. They would need to cough up cash and gain a bad reputation along with it. It is easier and cheaper to spend our money to keep the place up.
Just my thoughts on the matter. What are yours?