There's a significant difference between not developing additional land vs re-selling an already developed property. I don't know what land goes for in those areas but 3-4M Vero + Hilton Head points potentially re-sold in 2042 for $250+ each is a nice chunk of change. And once sold, there's almost no overhead for Disney since member dues cover operating costs, taxes and upkeep. Disney actually profits from breakage inventory. And there's some intangible benefit to
DVC having destinations outside of the two theme parks. More than a few spouses have been swayed into buying after hearing that "you don't have to go to the parks every year; there's also Hawaii, South Carolina and the Florida coast." (RCI fills that role to some extent, but having native resorts owned and operated by Disney is more straightforward.)
Vero isn't much of a tourist area. The beach is lined with private homes. And there are even some undeveloped plots. The land Disney sold off nearby is being developed as residential. Not sure it's fair to conclude that buyers would line up to acquire it from Disney. Hilton Head would probably fetch more interest, but its value is diminished by the fact that the hotel isn't located on the Atlantic coast.
EDIT: The sale price of the unused Vero land wasn't disclosed but before the sale in October 2019, the full resort including both developed and undeveloped land appraised for $4.6 million. If you rework the
point charts to get about 1.5M points and sell them as part of a trust beginning in 2042 for ~$250 each...
https://dvcnews.com/other-resorts/vero-beach/4607-disney-completes-sale-of-undeveloped-vero-land