How would frequency of travel come into it?

I would assume (without relevant knowledge) that it would be hard to argue that a one-off business trip in sardine class is a dangerous hardship for the majority of employees. After all, zillions of people buy such tickets voluntarily for leisure travel. But it legitimately is for some, and might become so for more with sufficient repetition. How should one draw the line?

I used to work for management consulting firm where weekly travel was the norm, and those (like me) who travelled intercontinentally frequently and on an ongoing basis did so by at least semi-conscious tradeoff. And in recognition that flying was a regular part of the lifestyle, J class > 4 hours stayed the norm throughout the economic cycle in spite of any client pushback. I'm pretty sure that I and others would have walked out the door if that policy changed. And we knew what we were in for when joining, and when deciding to stay. I've always felt for those who landed in positions with similar travel requirements, but more stringent policies, as a consequence of non-deliberate career evolution, say at a long time employer where they felt "no" is not an option.