Yeah - my husband’s grandmother used to travel cross-country fairly regularly. She would buy two coach seats (cheaper than a single first-class seat) to have a bit more elbow room. Note that this was before airlines were the cattlecars they’ve evolved into, and she was a petite woman, so it was really just for comfort.
I’m tall, and wider than is ideal. I haven’t flown in 7 years, but I’d consider purchasing an extra seat for the same reason - but these days the airlines often won’t let you reserve a seat in advance. So even if it’s an airline that requires larger people to buy a second seat, who’s to say they’d even guarantee that seat was next to the one I was occupying 
(like that person with the broken knee).
Anyway, I’m with your wife. If it’s less than 2 long driving days, I’m driving not flying.
Re families being seated together (which I think was what was happening with Coulter): We flew Southwest Airlines back when the kids were 10 and 7 respectively. The outbound trip was on a half-empty flight so we had plenty of seating choices. The return flight was jammed, the airport was jammed badly enough that with (supposedly) plenty of lead time we barely made the flight. Being forced to sit away from the kids was a very real possibility - would the airline have moved people around to help us out? The only thing that saved us from THAT mess was I’d had the forethought to break my arm while on vacation, which got us into the “needs extra time” preboarding queue :D.