As someone who loves crafting, the idea is great. BUT, realistically, it's not possible to do this. Not for reasons that involve needing to code it, but for reasons that involve the consumer. It's easy to come up with "good" ideas on paper, but it's a completely different matter to execute and QA them. An idea, alone, isn't even worthy of being considered 10% of the total work.

For starters, we all know that RMT have a strong presence in this game. Making a method they use for their business cater to endgame significantly is one of the worst things you can do. We don't need a repeat of the FFXI economy, where nearly every facet was manipulated by RMT. We also don't need them to be eating up DF queues 24/7, which is probably a minority now (but would be a significant presence if such things were to happen), or bot camping the strongholds. Source of money from sellable goods? Particularly endgame ones? You better believe RMT will be all over that. Position hacking is obviously easy to do, so no matter the difficulty of something, it can be bypassed. Apart from the fact it's in this game, it's something that was easily (commonly) done in FFXI and other MMOs.

Then there's also the dev philosophy on the direction of the game. I'm pretty sure that Yoshi-P said he didn't want crafting to be too heavily relied on with endgame. Whether or not this being the reason, it "forces" players to take up crafts to be competitive. Relying on crafters for materia is simple enough, but relying on them for endgame gear to be made is a totally different matter. One is essentially consumable, the other is not.

Again, good idea on paper, but reality is not quite so easy to work with. You pretty much have to look at things from the worst realistic possibility to understand why things may and may not work.