Keeping yourself busy with rewards what doesnt have any value and content what you alredy beted several times before but this time you can do it with bigger numbers will never keep players desire to play. Maybe if leveling wouldnt take 8h from lvl 1 to lvl 60 players would not run out of content. Maybe if finishing dungeon would not take 15 minutes of mass pulling of half of instance people would not run out of content. Maybe if raids would not be beatable by pressing single button people would not run out of content. Maybe of world content would present actualy threat and things could kill you people would not run out of content. Maybe gear would be actualy rare and not riking from the sky people would not run out of content.
Maybe if game would not be this acessible and coviniente garbage for every 80y old grandmas what have 10 minutes a day to play game people would have fun and content would last more than few hours.