I don't think Diablo's downfall was its AH, but the total lack of "endgame" content. There was nothing to do once you were geared up except run the same content endlessly (and it was literally identical each time-almost nothing changed game to game).* The AH accelerated the problem and made it painfully obvious to players because everyone got to the end game faster through buying gear. AH or not, the problem was always there, and would have ultimately motivated me to stop playing; it just happened faster. The lesson for me is that the content you play end game really has to give you a reward beyond gear. Community badges, titles, collectibles, and other rewards keep players playing once they get the set-up they want. I loved the "skull" system in Reach, and the daily challenges were enjoyable scenarios that kept me playing and improving. It also kept me playin for the pleasure of playing and overcoming a challenge. In games with items (like Destiny and Diablo), we still need challenges and rewards that don't revolve exclusively around better items. *Also, no skill trees were a problem. But that's for another post...