If he adds the expiration of the points, renters will come to believe that they can be rebooked for any reason before that day. His office will field a 100 calls a day from now on that has some variant of, “but the points are still good for eight more months; can’t you just work with me?!” - and that will frustrate his business model. There’s a reason he stressed no refunds, no changes and disclosing the expiration date of the points just adds bad feelings for people who need to change their dates.
Regarding 50% up front, not gonna happen. If anything, he’ll need to go to 80% to entice some owners back. David’s model is in danger enough without him implementing policies that would give him access to fewer owners. If he tried 50%, he’d never find enough owners to even make this voucher plan work, much less to actually make money in the future.