HQ mats still hold a place on the MB as a commodity because it gives people a peace of mind. Kind of an insurance policy if you will.. Not saying it's necessary, but if it's 20-30k extra most people will pay the premium to ensure they are protected from horrible RNG.
Also, reason wolfram ingots don't really sell is because people who need them can already make wolfram ingots. What they should've done to encourage cross class material trades was to diversify the materials for crafts instead of having GSM use gold and platinum for 99% of things, and BSM/ARM use darksteel/wolfram for 99% of things etc.
All that aside, there are people that make most of their gil by gathering and selling mats. Gathering is their main game, and they refine the mats to give it a little more value etc.. It's a lucrative portion of the market.
Besides, we're not discussing the viability of hq mats for 2-3 star crafts, we're discussing the value of the craft if one were limited to one or two of them.
To somebody who makes their gil by the means I described above, if they were forced to specialize in one or two of the classes, their market basically shrinks by more than half. Can you imagine trying to make gil by only selling rose gold nugget only? then move on to rose gold ingot then gold ingot into platinum ingot? That would be a very slow progression, if even possible with all the competition to come with it. Of course, you can say to diversify into gear, etc. But that argument just brings us back full circle..
This will apply even to people who sell gears. Of the people who sell artisan's gear on the MB, who only sells artisan's sandals/grinding wheel/spinning wheel? (taking CRP as example) nobody, if someone sells one artisan gear, they usually make and sell the rest of them too.
Such a change will affect everybody, think about how many types of final product you deal in.. It's usually 3-4 classes or more. Now, eliminate them by classes until one or two remain. Think about how profitable that'll be for you to all of a sudden cut your wares by 1/4th.