OK, my tuppence worth - sorry if it's been stated already, I didn't read all of the preceding 22 pages as it's end of FY in my job and I've had a late day, with plenty more to which I can look forward...
My wife & I fly annually from Aus to Dublin to see relatives. We fly economy and are both in our 50's. I don't mind the flight too much and we try to get seats where there's a bit more room, e.g. the pair of aisle-window seats at the rear of a 747.
Personally I would not pay >= $4K more just to have more comfort for 24 + 24 hours. I would still get jet lag and swollen feet. My wife is getting to the stage where I might send her business as she would benefit from the comfort and not require a chiro at the far end.
I think age has an influence too: younger = more flexible physically + poorer and older is vice versa. Sorry about the huge generalisation!
I also think that on a Aus - Europe flight, for example, you can alleviate some of the hassle by booking into an airport hotel for 12 hours in Asia and having a sleep, a stretch and a shower. Not always possible if time not on your side, but it breaks the journey.
Finally - nothing to do with class! - I always ensure that when we return to AUS, we land at the start of a weekend and take the following Monday and Tues if possible off work. Wandering around your home town when everyone else is at work, also knowing there's only a 4 or 3 day week ahead, is more relaxing than any swami, meditation or anti-stress pills.