Price-bridging would better be described as raising the original value of a ticket to current gate price value. Then the new upgrade is based on current gate price. It's not so bad to eat $11 (your example) for a single one-day ticket. But many people have 2-day tickets because of FD. And they have them for a family of four. So that price difference can add up. There are others who buy tickets at a discount from
UT and keep them for future trips. They may have up to 10-day tickets. The price difference on these tickets can be quite a bit, multiplied by an average vacation family of four.