So you want it to follow the D&D / Neverwinter model of farming for keys to have a chance at getting something random and possibly useless after clearing nearly an entire dungeon full of stuff. That would work if they enemies dropped gold and loot as well. With the way the game is set up now, making it so you have to farm loot to buy keys to farm loot it would break down rather swiftly. There's already issues dealing with the chance that the game kicks you out, imagine how pissed someone would be if they farmed to acquire a key and then finally get to a chest to have it crash before they get the loot.
It works in that particular game because you have multiple ways to acquire loot, gold and consumables; in DA gold or rwc are the only ways to acquire upgrades. It would throw a monkey wrench in it for anyone that wanted to start the game behind the power curve because people already purposely skip giant chunks of the mp maps and pots to skip through to the end.