When the advantage is massive enough it starts to feel, to the player, that it is necessary. You could get artisan's weapons/helm without having any other crafters at 50
(or even 15), it would of just been a massive pain, and most people wouldn't seriously suggest it as an option.
That's a poor argument and you know it, opportunity cost is a thing. One person spending a million gil's worth of materials for the same reward as the guy that spent a few hundred thousand worth is broken. "Hoard more materials!" is just completely missing the point.
I never said all materials should be obtained the same way so I'm not sure where that is coming from, but yes, they should adjust the scrip rewards based on the material difficulty. If they want to make adamantite stuff to be more expensive to make than any other comparable crafting material, fine, but you should be rewarded for that expense.
If you're bringing up opportunity cost, you have to consider the savings of time and gil from not getting other classes up to 60. I would say the adamantite users are rewarded for their expenses: they can make and sell the new two-star main hands and off-hands. These items are likelier to be sold on the market than some of other crafted items available.