See, your claim is that fuel cost is the sole reason they do this. I'd argue it's obvious they'd do that even if all fuel was free.

> Aircraft speed as barely budged since we transitioned from turboprop to jets.

Ah, it's again one of those nonlinearities you seem to have trouble with. See, the cost increase is, again, not proportional to speed. On top of the quadratic scaling, you have a very nonlinear and steep (not-proportional!) increase in drag coefficient. So, when you look up that what I say is true, but you want to weasel your way out by saying 'it's not by much', you're missing that the impact on drag (and fuel) is substantial.