But the thing is there is still a trading economy in the game. It will still exist, it's now just unofficial. In diablo 2 you traded items by joining chat rooms and spamming your wares or joining message boards. You then traded by arranging a game together and trading in person.
Actually no; when they made the auction house they reduced the quality of items that dropped. This is evident in the console versions, which have vastly different "loot tables" and no auction house. So they didn't just make it convenient for players, they tailored the items that drop around the fact that anyone could sell and buy items.
For the small group of players that always did trading that's probably a good thing, but for the vast majority that want the game to be Diablo and not the auction house it was a pretty bad change to the core game.
As a new player, I really liked the AH: Every single thing that dropped for me was either completely useless, or orders of magnitude worse than what people were throwing away on the AH for a pittance. (Sub-optimal stats went for cheap.) So, I could basically always find something that would be a significant upgrade every few levels.
If I had actually been seeing drops that were any good (due to a non-depressed drop rate), I'd have not felt the need for it ... but using the AH basically quadrupled my character's effectiveness. Using it was definitely part of "playing the game".
It made leveling great: You can freely vendor/DE almost everything you pick up if you want, and use the cash to buy things on the AH. Profit! At the end game, though, I can absolutely understand why that would kill the fun.
They are just removing the convenience now.