To me, there's a significant difference between a trip where you can get up at home early in the morning but get to bed in your hotel late the same evening, and one that is even a few hours longer. There is a much less significant difference between that and one a few hours shorter - if i arrive in the early evening, i'm still not going to do much with the day. The next significant difference is when a trip is short enough to do the outbound and return legs in one day, and still have a useful stay. Concorde wasn't quite fast enough to that.
Taking a 5 day longer trip to save $1800 is equivalent to getting paid $360 per day. If your salary is less than $90,000 per year, you'd be better off taking the ocean liner.
My previous contract had these terms, although I rarely claimed all the time off to which I was entitled. It seemed a bit ridiculous when I'd already bent the travel rules as far as they could go, in order to take a holiday abroad after most trips.
https://www.peninsulagrouplimited.com/guides/travelling-for-...
(And, yes, business or first class that you actually pay directly out of pocket for is priced like a luxury good. Of course, you can pay a lot more for the cabin too.)
You can get a taste of this when you look at the cost of long distance rail— for example, a trip like The Canadian (four days in total), which VIA understandably sells as a cruise ship type experience rather than practical travel:
https://www.viarail.ca/en/explore-our-destinations/trains/ro...
Obviously some of the cost difference is just the massive economy of scale enjoyed by airlines, but at the end of the day there are certain fixed costs associated with being in their care for such a long period of time. Even with a cheap ticket where you don't get a sleeper bed and have to buy/pack your own food, they still have to staff the train that whole time.
I did take the overnight from London to Edinburgh a year or two ago. It was actually pretty convenient but it still almost certainly cost more than flying.
Honestly, I'd guess that depends on your job function, whether you can work effectively remotely, and the cost of the ticket.
As a remote employee, I can totally see justifying it to my employer. It wouldn't be typical, but if it costs about the same and the trip there and back would involve me getting as much or more done than I normally would... I'd definitely pitch the idea, and don't see any reason why it would be out of the question.
In fact, I'm making a mental note to investigate the cost when/if I need to attend a conference or something in Europe.