When you "CAN" have every skill, and people will, you eliminate the need to buy from other people.
While I agree with your statement and it "could" happen in reality, I shall give you my viewpoint as I had everything leveled in 1.0.
If I wanted to make items for spiritbonding / mass crafting sessions I wouldnt go out and farm the base materials just to turn it into another item which I would then use as another base item for the required armour, It was just too long a process. I would run to the retainer market and buy as much as I could to save me time and only craft what I really needed.
If it became apparent that said items were not available, which was rare even towards the end of 1.0, and I could make good profit vs time It would take to get the items, I would farm for those items and then sell some of it on for profit and use the rest for crafting.
People will always level gathering crafts, people will always level a / multiple crafts to make money. Some may stop at a level which allows them to make a profit on fast selling consumables. This happened in FFXI, it happened in FFXIV 1.0 ( Cannot comment on others as I haven't played anything else ) and I can pretty much guarentee it will happen in 2.0 as well.