For this example, let's use the Goldsmithing slots: If the necklace, bracelet & rings slots could only be filled with crafted gear (no more drops for those slots or NPC vendor gear to purchase), we would have 2 amazing things happen:
1) Crafting would be useful again: Rather, it would be useful for the first time ever. If every equipment slot can be filled with superior dungeon/raid drops, crafting largely becomes busy work. The end result is not worth the gil and effort put into creating it. I love the FF14 crafting system! It is so heartbreaking that I am making things that have minimal value and will not be used.
2) The economy would be dramatically fixed:
a) Crafters who do all the DoH/DoL jobs will be in high demand. They will be able to create gear for themselves and others. If they also start slapping different materia on the gear and sell it, they would be custom pieces. It would be so RPG! Tremendous work resulting in tremendous reward.
b) Players who do craft (but don't do all the DoH/DoL jobs) would need to spend money on raw materials, thus creating a windfall for gatherers. They would get to enjoy the crafting process and make beautiful gear that they can dye & wear in combat!
c) Gatherers would need to spend lots of money to buy crafted gear. But they would also make lots of money from selling raw materials in this red-hot economy, so they can practically get their gear for free! If they are willing to fish/gather their butts off, they will be rewarded with more than just gil.
d) People who don't gather and don't craft will finally have a use for their money! Many people in FF14 are sitting on millions of gil. They have nothing to spend it on that they personally like. But they will certainly like getting gear!
Crafters could listen to gossip and create new pieces based on the meta. Skill-speed necklaces! Piety rings! The money earned from crafting would be plowed right back into the economy via the Auction House, used to buy more raw materials to create better and better pieces. FF14 Crafters would start advertising their work outside the game. A hot crafter would get invited to the most exclusive FCs! The possibilities...