Dear Babe ...
I just saw some industry (airline) stats yesterday. Seems like SouthWest is making a profit when some of the larger carriers aren't.
I think it was on the Reuters wire.
Must be those itty-bitty seats and low fares.
I could go into humor mode here, but you present a very serious question about safety.
I'll do my best to answer.
The first thing I do when I get on an aircraft is check the emergency exits. Then I scrutinize the people sitting next to the windows (flight attendants handle the doors).
I tell you, with all seriousness, that my plans are made to crawl over rows of people, if necessary, to get over-wing exit windows open, well before the "official" safety announcement is given, which nobody seems to pay attention to, anyway. I feel reasonably confident that all former airline crew thinks the same way as I. And it's not to get ourselves out first. It's to help. That old training STICKS.
We once flew with a retired pilot, who sat next to me. We figured out each other's former occupations FAST because of the eyes skating around looking for the exits.
If I commit the sin of political incorrectness by putting morbidly obese people in the category of "physically challenged", I apologize in advance. It's quite possible that an extremely heavy person might not be able to get THROUGH an emergency exit window. Paying tor two seats wouldn't help at all in an emergency situation.
There are also the wheelchair people (who are without their wheelchairs when in flight), blind people ( I have a dear friend who is blind and flies with her Seeing Eye dog), the pregnant ladies you mentioned, even a young mom flying with a child or two or three falls into what I would define as a "high risk" category for evacuation.
Of course, the airlines aren't going to scare passengers by doing much more than the lame FAA-mandated safety announcement.
It may be of interest to know that cruise ships have well-honed plans in place to evacuate such folks. And they know WHO the people are and their cabin numbers.
Back to your first question, if people are very tall, arthritic, or whatever else, they just should not fly on airlines or in a class of seat where they will be uncomfortable. That's choiceful.
Dumbest thing I ever saw? A few years back, a guy on a 13-hour flight who'd just had a double knee replacement about three weeks before. Think of airpressure at altitude. His surgical wounds popped. He wanted to sue the airline because it ruined his cruise. I don't think he should have been going on a cruise in the first place.
I hope I've answered your questions and concerns. If I can add more, just ask.