They'll start with the same silicon design but manufacturing variability requires them to bin the chips based on functionality & performance, which is how you get all these different models.
Nobody would complain if Intel sold of 8core dies with 6 working cores as 6core CPUs. Disabling the ECC or AES-NI support just to create artificial market segmentation is an asshole move.
That is true, especially if you're in a position like AMD is where they can sell every single good chip they get into a high margin server part (which is why ryzen can be scarce). But that's pretty rare overall. As a process node matures, the amount of perfect dies will go up dramatically, but there will certainly be no price drop to compensate.