flight attendants are great at finding a place for oversized luggage clogging up the overhead bins. if the overhead bin is full sit your carryon on your lap and when they ask why it isn't stowed they will fix it or stow your bag nearby...
Which may be fine and dandy if you're in the back row of the plane, or you have no connecting flight. But I can tell you right now that if my bag gets put 20 rows behind me, I am not going to go against the stream of people to try and grab it. That would be terribly rude. And I do not let other people walk the wrong way down the aisle to grab a bag when they are getting off either. I barely fit down the aisle, both height and width, so good luck getting around me.
That's not to say you can't get around that. A month ago I was traveling and gave up my window seat to a Chinese couple so they could sit together on a long transcontinental flight. I moved back 5 rows. I didn't bother with a carry on for this trip because it was a long one, but their carry on bags stayed in the back. Even though they barely spoke English, I helped them find their bags and pass them 5 rows forward so they didn't have to wait to get off the plane. But in general, if your bag is in a bin more than 1 or 2 rows behind your seat, you're screwed when you go to disembark.