It's quiet simple really. Have everything in the game drop in game but make it very rare.
That way, people who don't buy it have a chance to get it anyway by regular playing and people who do not want to deal with RNG can buy it from the store.
There, the sub fee is still used for developing items that are obtainable in game - and also blizzard can make more money of them.
Can anyone come up with a good reason to why this is a bad idea? It's basically 2 paths of obtaining the item, and it would make both sides happy.
EDIT: complaints..not complains :/