People frequently have to either cancel or re-schedule a trip, and Disney tries to be very accomadating with them. We had a family emergency, and had to reschedule on 2 days notice (we actually were moving up the cruise as the later date had the emergency conflict that I had to be back for)...it cost us a small fee for "re-ticketing." Of course, that meant that a cabin was now open on our originally scheduled cruise.
People can re-schedule up to 60 days in advance with no penalty, after that, there is the small re-ticketing charge. So...it is totally possible that you will get a room from the wait list.
Also, as the cruise date approaches, if the ship isn't sold out, Dis may offer someone else an upgrade in order to sell you the cabin. It is better for them to sail with 2 happy families than to go with an empty cabin in any category.