The single jump seat option on Spirit planes was a cost savings idea which eliminated the jump seat and O2 mask and it’s associated maintenance. Same with brake fans and foot warmers and a few other options. Some of the original 319’s and aircraft acquired from other carriers have them.

As far as future orders, changing the spec usually costs money after the order is placed, even years out.

Most likely the bean counters will figure out which costs less and go that route, which may include taking differently equipped planes from each order.