Right, but the point is to move the needle on the margin. There are a lot of people who always do carry on only just to avoid the fees. If, say, 20% of people are just avoiding fees, then that would likely solve the problem. It's not like there are dozens of bags unable to be housed in overhead bins. On most flights, it's none or just a few.

The point is also that there needs to be some excess unused capacity for efficiency to make it faster to embark and disembark. If people need to take a minute to hunt for a bin, it can hold up the entire process.