Reserves cannot drop days. A typical base might have around 20 reserves a day with a “min required” number of 10. That means that, as long as there are 10 or fewer trips in open time on that day, it will be green and authorized for drops. If there’s more than 10 open time trips, then there aren’t enough leftover reserves and it’s red - can’t drop or swap out of that day. It goes green again if someone picks up a trip. The month must start 75% green and every day must have the same “min required” number. After that, as lineholders make trip adds, drops, and swaps, the grid fluctuates substantially day to day. So no, not everyone drops to zero, but not everyone wants to. But almost everyone has a *ton* of flexibility on their schedules.

And since sometimes even people with “good” trips want to drop certain days, there’s usually some decent stuff in open time. Then people pick it up and those days go green allowing for another person to drop. Etc etc etc