While it's always been quite popular, I agree with the OP that booking early December at 7 months has been getting more difficult over the past several years. Demand for that time has definitely increased. Some of that is due to more DVC members wanting to travel at that time. But IMO, most of the increased demand is coming from the general public who are eager to rent DVC reservations.
More members are choosing to rent and their customers are catching on to the need to book prior to 7 months for the near park resorts. The brokers are making it relatively easy to turn unneeded points into cash. Note that the brokers are paying a premium to the near-park resort owners who are willing to rent during the home resort priority period. Even with that, they don't appear to be able to satisfy the demand for some times of the year. Obviously, if owners choose to book during the home resort priority period, there will be less space for those who must wait until the 7 month window opens to book.
The lesson for us is to BUY WHERE YOU DON'T MIND STAYING - ESPECIALLY IF YOU PLAN TO TRAVEL DURING POPULAR TIMES. The odds of successfully changing to a near park resort at 7 months are no longer in our favor.