This difference won't really affect me. My personal item is a large belt bag that's well within the new limits, 12x8x6 plus a bottle carrier. It easily fits in the underseat of the 800 series interiors with plenty of room to spare for my boats, er, shoes.

I'd be more upset if the overhead limits shrunk. My rollaboard is under the current limit, but not by much - 22x15x10 including wheels and handles. It fits easily in the OH wheels in, but since it's at the 10" height limit, I'd have to go hunting for a smaller bag if the limit ever shrunk.

In my experience, I'd say it's closer to three or five percent who abuse the carry-on restrictions. That is, one out of every twenty to thirty pax breaking the carry-on restrictions in some way - either with three items, or two items that are both too large for the underseat space, or items that exceed the max dimensions for overhead stowage.

The most extreme case I can recall was a BWI-MCO flight around 2012 or 2014 where there were something like a dozen bags that had to be gate checked (along with all the strollers), because the overheads were full, and every overhead had one or more items were too large to fit in wheels-in or wheels-out and had to be turned sideways. I remember on that flight someone boarded with a rollaboard that had to be 26" long; a 'carpet bag' style duffel of nearly the same size; and a shopping bag that looked to be about 22". These three items took up an entire OH compartment, displacing the items of everyone else in three rows of seats. Naturally, the owner of those three items shoved them in haphazardly, leaving later pax to re-arrange them in an attempt to fit anything else in the bin. And of course, the owner of the items also sat nowhere near them - they dropped off the bags in the second OH compartment on the starboard side and proceeded to sit much farther back in the cabin. Thankfully, cases that extreme are rare.

But on pretty much every flight I see people bring a rollaboard plus an overstuffed backpack aboard, neither of which would fit in the underseat, or two overstuffed backpacks, or an overstuffed backpack plus a duffel of some kind. Sometimes they'll also have a third item, such as a purse, fanny pack, or shopping bag, that does fit in the underseat, but their two larger items both go into the overhead.

I have seen a number of people bring a rollaboard plus a smaller backpack that seems like it might fit in the underseat, but when they get it down there it won't actually fit completely, and protrudes halfway into their foot space. That leaves their feet in the aisle or the foot space of the pax next to them, and presents an obstruction in case of restroom visit or emergency evac, especially if they're seated in an aisle or middle and other pax need to clamber past their protruding bag.

Again, I see this kind of envelope pushing in about one out of every twenty to thirty pax, or between five and eight pax per flight. That doesn't seem like many out of 175 people, but given the limited amount of overhead space, five to eight extra bags going in there instead of in the underseat can force five people in the C group to gate check bags full of valuable and/or fragile items like electronics.