I agree, a mining world would not only ruin the economy but would also take away the "fun" of mining. There are still plenty of materials available, even in areas that look like they've been mined already. There is no better thrill in Minecraft than finding a jackpot of materials and KNOWING that someone else missed them.
Two days ago I was in an area that looked completely mined out. I'm talking about a horizontal shaft every 5 blocks... Guess what? I cleaned the areas between their shaft and found myself over 10 diamonds, over a stack of iron and redstone, and half a stack of gold. So just because an area looks mined it doesn't mean there isn't anything to be found. Build yourself a ton of stone pickaxes, a couple iron ones (for when you find stuff), and just go crazy. Trust me, there are a TON of materials that people have missed, even right beside the spawn city (where I located 12 diamonds and another stack of iron).