I don't think anyone is saying that... They're saying crafters aren't buying stuff.
Why would anyone who crafts opt to buy something from someone else, when they themselves can likely make it for a fraction of the price? They wont, and they didn't.
If gear is worth buying again, then this would only increase the number of people who are buying, because all the omnicrafters (myself included) who can currently make everything would no longer be able to do so, we'd have to rely on others to obtain certain items. More people buying is only a good thing for the games economy. Less supply would probably be a concern if I didn't think this game already had too much... The i180 stuff has very little supply, but that's because of the rarity of materials (and worthlessness of the gear itself) more than anything, perhaps your server was different, but I remember stuff during the 2.0 cycle being routinely overstocked...
2.0, I would say, had high supply and low demand. High supply was a result of everyone being able to make everything. Low demand was a result of the same thing; I'm not going to be adding to demand when I can produce everything myself. Specialist recipes corrects this. It lowers supply while raising demand.
That's why I have an issue with Specialist skills. They were meant to make it easier to get into crafting, but they haven't done that at all. SE really needs to address that, since it would hopefully get more people into crafting... The main hurdle with crafting at the moment is that you need all eight leveled to really do anything with one of them, if Specialist skills corrected that like they were said to pre-launch, I'd imagine more people would get into crafting.
There is also potential for a general shift in gear to push more people into crafting... With crafted gear being stronger in 3.2, along with materia being even more desired with the ability to meld to tomestone/raid gear (plus Diadem seems to be making it possible to actually Spiritbond towards tier V materia), people are perhaps more likely turn to crafting to make Gil. Certainly most people I know who don't craft (yet loved crafting in XI) don't do so because Gil is fairly meaningless in this game, with there being nothing really to buy...