Someone upthread had the very sensible suggestion that if you need to leave with other people in your row, you should wait until everyone else has left the plane.

Excluding children, for whom exceptions can be made, I'm not sure I understand why this is much of a problem. Does it really matter if you walk fifty feet without being next to your travel partner?

It might do, you are often not allowed to wait around as you walk from the plane to the terminal building and to passport control. You may be in different places in the queue. Now say one of you goes through to the first place you can wait, perhaps after passport control and before baggage collection. The other person doesn't turn up. You don't know why? Wait another 10 minute? Did they go through baggage area? Passport issues? Where do you meet now? You can't help them. Granted mobile phones, but not being split up is easier.

Now friends sometimes end up in different seats, and you can probably get away with waiting somewhere, maybe on the plane itself to meet up, but if everyone is in the same boat it might be chaos to do that, and there is more incentive for airport staff to stop you.

Well sure, I don't think anyone was suggesting that no waiting area is provided to meet back up til you hit the sidewalk. I thought it'd be clearly implied that there would be a spot to reunite after exiting the jetway

Even from a purely utilitarian perspective where efficiency is the only concern, if you force groups to separate then you are just going to clog up the jet bridge with people waiting for their loved ones to exit the plane. If there's no jet bridge and the passengers are getting on a bus to be taken to the terminal it would cause even more chaos.

I don’t quite understand the desire for friends or family traveling together to exit the airplane at the same time and hold up others as a result.

Airports have waiting areas between arrival gate and passport control, probably precisely to let passengers traveling together catch up with each other. Even the worst possible airport would have a bathroom, and toilet entrances are implicitly a waiting (queueing) areas.

Unfortunately, introducing any sort of rigid structure and order into airplane disembarkation process sounds like an impossible task.

Seems to me the real problem is people in the same row who don't board together, since frequently someone has to squeeze back into the isle and block things until their neighbor is seated. Airlines know who booked together, couldn't they optimize the boarding groups flight by flight? If some people booked together and sit together, give them all the earliest boarding group that any individual member would have had.

Very good point. And unlike boarding, trying to handle exceptions first or last doesn't really work because people are physically blocked from leaving their row. I've figured we'd never do this for cultural reasons, it's just something I ruminate over while traveling alone, staring at the empty aisle of a half-full plane.