[bluGill]

They can't upgrade people on an empty flight without upsetting the balance of the plane. I've been on flights where we didn't leave the gate until people downgraded so the front wasn't overfull. (I was the only person flying economy, first and economy plus was full so everybody was in front of the wings)

[lordlimecat]

I'm having trouble believing that the margins are so close that the position of a 200lb mass on a 68000 lb plane is a significant factor in anything. If true it would suggest that using the lavatories would be a significant event requiring coordination with the stewards and pilot.

[quacked]

30*150 = 4500 lb. There are cases of planes crashing because everyone rushed to the front or the back to avoid smoke.

I know it's not the same as a commercial airliner, but I used to teach hang gliding on the beach and we'd sometimes get a little packed sand in the back of the keel (which is the name for the central bar that the wings were mirrored across). The total weight of the student plus the glider could get up to ~280 lb, and the amount of packed sand would be less than a pound or two, but the flight characteristics of the gliders would change drastically, to the point where we could barely get them in the air.

As another commenter notes below, the few people moving around the cabin is less about control ability and safety than it is about fuel efficiency.