Yes but they restrict availability. You are suggesting they take that restriction away. That's the problem.

Delta limits their expense by restricting inventory.

If they give up seats that they would otherwise sell, yes. They lose the entire difference between the economy ticket and J. Which, for some routes, can be that high.

When I say 'up to $14k', surely you understand that it is possible for you to find a cheaper fare without my statement being false. But maybe not, nothing much else you say makes sense.

Your whole premise is looking at things from what you want as a passenger, it makes no business sense whatsoever.