And there is zero logic behind your second obviousm, as your premise is already wrong. The reason they start with business class is because you can charge more per seat. Seems pretty obvious.
You're right that they start with business seats because they cost more. Business class seats cost 3-4x time economy while occupying less than 2x the space so the cost of fuel as a percentage of the ticket price is lower. It might not be obvious to you but I'm happy to explain it :)
[0] https://www.mckinsey.com/industries/travel-logistics-and-inf...
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.