I kind of look at like "first come first served." If you are in the last set of passengers to board, you know there may be no overhead space left. I don't think people should be forced to place things under the seat if they chose not to. Of course it's kind to do it, and I always do because I frequently get stuff from my bag. But, some people have leg/knee issues and need to stretch legs. Plus if you are in a middle seat you are already squished in so the leg space may be a necessity. It's really no big deal for the attendant to take bags when there is no room, they wheel out a cart and you get it right as you exit the flight. I have had to do this many times.