Allowing low-ball bids and then not accepting them if space opens up just annoys people. They will learn that there was empty F space, they had bid, and yet they were not upgraded. Far better to establish the minimum amount upfront.

For many, the need for flexibility disappears as train time approaches. It's one thing to hop on a delayed train at the last second once in a while. But, 1-2 hours prior to departure means that most people have a set schedule.