The thing with a duffle bag is that they are squishy. If people put additional stuff in the bag afterward, a sizer beforehand is useless. In many cases, though, it's not that the bag itself is an illegal size, but that everyone has at least one bag that size, and many people have two. To me, that's a big part of the issue--enforcing the ONE bag, and only ONE personal item (that HAS to fit under the seat).
.
