However, what really is the magnitude of the rental market. I suspect it's a very very small percentage of
DVC points that are rented out, thus not really that large in magnitude of those wishing to rent (the waitlists at the two biggest brokers aren't 100s of reservations long even). Also those that are renting DVC points are very savvy with their money (i.e. going the cheapest route for Deluxe accommodations) so they might also be the first to start contracting on their vacations if a downturn happens (they are behaving rationally currently so could assume they would behave rationally in a downturn).
Though personally I subscribe to the idea that the current bull market has led to an increase in rental demand. However, a recession won't necessarily lead to a decrease to rental demand. The reason I think that isn't because the current renters would necessarily keep renting, but because the public education is out there on renting DVC points and cash guests wishing to keep going to Disney might convert and fill in any emptying demand left by those deciding to no longer rent.
Though it is important to note that the last two large recession (2008 and 2001) Disney cash resorts were operating well below fully occupied (as was the parks).