Nvidia wouldn't leave money on the table dropping the price of the 4090. There would just be more supply of 4090s at the same price. A card manufacturer selling them below MSRP would get immediately sanctioned by Nvidia.
Even a monopolist is constrained by supply and demand. If they could sell everything they make as a 4090 without trashing their margins, they would. The fact that 4080, 4070, 4060 lines exist means they can't.
Right. And the fact that a BMW 2-series exists means that BMW can't sell all the 7-series' that they want.
They're cutting down 4090s into 4080s to fulfill a demand for a cheaper chip, while still supplying their premium option. Your fanciful world concept of things being sold for no/minimum margins is just that: fanciful.