Again I'm not talking about "getting rich quick" in this senario. I'm talking about you recuding what everyone else can get for their hard earned goods. It creates an uneven platform which punished people for not going those classes. Plus your suggesting giving this to pretty much one of the only classes that is still useful in the current standing of things (smith) which only further serves to punish those that don't pick the class.
In addition your idea to restrict iron + pickaxes to miner is an even worse idea considering that give miners the monopoly on redstone, emerald, gold & diamond. Which is especially idiotic considering your suggesting to move to a gold only economy which would mean miners are the only ones that can produce wealth. Not to mention the only reasonable way to level certain classes like smith, enchanter, engineer and some of the others is to MINE. You'd be removing the only viable way of leveling over 1/3 of the professions.
I have actually seen restrictions like this work. It's not theory it's practice. We need to feed the desire to acquire.
Economic interdependence is the foundation of every functional economy. The desire to acquire is the locomotive of trade.
If you can satisfy your desire to acquire through only the value you yourself can create, there is no need to interact with the community. So what if only miners can get the resources necessary for currency, they will need to purchase their tools and sustenance from someone. So what if they have all the redstone, what the hell are they going to do with it it they can't build anything?
If your worried about a currency bottle neck buy gold at the exchange for 7.5 and allow them to convert it for 9 this would create a profit potential and make trading with merchants favorable but not necessary.
For a system like I'm suggesting to work you would need to make being a combat class and a merchant impossible. That will limit who wants to be a merchant, as they would have to chose between running a business and being a great fighter. They could have some limited combat ability but nothing substantial.
If you can't find another way to level engineer, like say experience for crafting and placing crafted materials related to their profession, give them iron picks, not diamond and have drops less for professions other than miner.
In order for an economy to work we need to make the most profitable activity for any class to be engaging in the things only that class can do. For a miner mining, for an engineer building, a farmer farming, a merchant trading, and so on down the line.
A system like this can be built. I have seen it work. To envision it though you need to stop thinking about how you've done it in the past, or how it is under the current system, and start thinking about what makes the most sense moving forward, and how to get there. We are talking about massive overhaul are we not?
A system of economic interdependence, driven by the desire to acquire, has been proven time and again to be the most stable, most productive, fairest, and most viable economic system.
Maybe I have the wrong idea on how to get there, but you certainly won't get there dismissing ideas as idiotic, that is an emotional reaction. We need logical thought. Only through rational, logical though can we manipulate the emotional drivers of others. Which is what you really need to do to keep people logging in. Emotional manipulation through the desire to acquire, and the feeling that others are counting on you.