Most people have a line that they are unwilling to cross. We found that when the nightly fees approach $100 and the campsites are "nuzzle to muzzle" but right on a beach, then the benefits are harder to accept.
On our first winter trip, we didn't make reservations months in advance except at a couple of places where we knew we wanted to be at a certain time. After New Years, we got real loose on which way to go and found that southern Florida was either booked or too expensive, so we heeded north and west instead. IMHO, the "sun belt" is going to lose a lot of business because once people realize that the rest of Florida is just as nice (or nicer) just not as hot, people will stop paying the big bucks to stay south of I-4.