Shortening the quests would indeed likely help resolve addiction but I think it ultimately also damages the game's attractiveness to hard-core players. Casual players who spend small amounts of time playing, when they have a moment in their busy day to do so, prefer shorter quests. It makes it easier to feel like you've accomplished something in the little time you had available. But players who have more time than money tend to disagree. Players who have nothing better to do with their day than to spend the entirety of it playing online games prefer those long involved quests. Making the game too easy risks alienating the hard-code audience and that would damage the company's bottom-line. A balance of both types is the best way to go financially.
Game addiction is a problem. But I don't feel that it's the game developer's responsibility to tackle it. Psychological therapy might be a better solution for people who can't hold a job or friends because they're addicted to an online game.