There have been times when I've had to take 3 economy flights in a row to get to a destination, travelling over 24h straight.
Travelling long-haul in economy if you're anything near 6 feet tall is absolutely fucking miserable. But not everyone has the luxury of easily getting another job, so what can you do?
The only upside is that after doing it long enough you gain status (but it takes a looong time flying economy), then you tend to get free upgrades a lot more often, and you can use your airlines to get a free upgrade on the rare occasions where the moon and stars align and the airline says there is availability.
I have the same Premium Economy rule nowadays, but I now have the luxury of being able to do that; earlier in my career, I didn't.
But yes, it’s about how often you fly. Once or twice a year on a midhaul day flight like Uk to east coast or Mid East in a good economy plane (qatar a350 for example) is fine. flying 30 times a year then just say no. It’s not worth the damage to your health.
IOW: it is absolutely the airline’s fault. And one of them, Spirit to be exact, is pushing the boundaries even further. Is there a limit to how far this will go? Probably not, left on its own.
And no, my job doesn’t ask me to smoke 40 packs a day, amazingly, and the two things you listed are not equivalent. It was, at one point in history, not unreasonable to ask employees to fly to other locations on company dime. It is slowly becoming unreasonable because there are no checks preventing this and people literally can’t just stop flying, this isn’t a real solution, the world would collapse with the absence of air travel, we can stop entertaining this thought.
But do keep yourself hydrated and go for a walk around the plane once in a while, it helps.