You could be SOL. (period) The Thursday flight already has passengers booked. Spirit will accommodate you on their next flight that has available seats. Your choice may be to cancel your trip or accept a refund from Spirit and find alternate transportation.
Travel insurance might help. Read the terms carefully. Trip delay benefits are generally limited. They're intended to help pay for your hotel and other expenses while waiting for your airline to accommodate you. There may be a few policies that will pay to fly you on another carrier
but most policies don't.
JMO but I don't think I'd use an airline like Allegiant for a winter vacation uless it was an "extra" trip I was prepared to cancel or re-schedule.