The high-volume low-margin economy customers keep seats filled to prevent wasted potential space. On most commercial planes, flight is only profitable if nearly every seat is filled.
No. If they could fill the entire plane with business/first class seats and sell out >70% (maybe even less) of it, you bet they would.
The only reason why economy class exists is because they can’t. But the demand for more premium travel is steadily increasing, which will lead to shrinking economy cabins.
So yes, they do need to fill the space. But I wouldn’t say that they need the economy passengers.
> No (...) The only reason why economy class exists is because they can’t.
in other words, in nearly all cases they need both
You also contradict yourself saying they only profit from business class but at the same time they can't profit from business class because there is no enough demand for it.
Your statement doesn't make sense and what the poster above you said is right - they need both and that's the reason there are both.