Fast forward twenty years, the resorts have stopped selling Timeshare. It doesn't make economic sense for the resorts to accept an exchange request from RCI.
RCI keeps the exchange fee, and resorts use up accommodation they could use to rent out privately through a holiday website or tour operator. They are potentially losing thousands in revenue by doing this; hence, why the resorts hold back their accommodation from RCI to use for themselves.
So when you contact RCI for an exchange, and they tell you "sorry, it's not available", what this means is there is no availability through RCI. If you contact the resort directly or through another accommodation provider you will be able to book.