WoW is monetized via subscriptions, thus they have a strong incentive to keep people playing for as long as possible. What *isn't* possible is building enough content for MMO players to keep happily consuming it. Remember the first two weeks of an expansion? This would be that amount of new content, fresh for every two weeks across the entire two year expansion's lifetime. Since that is impossible, they focus on repeatable content.
This isn't the only MMO monetization model, but all the alternatives have perverse incentives where the developers deliberately create problems only to sell you the solutions in their cash store. Even the good ones do this, like ESO and Guild Wars 2. Subscriptions aren't great, but they are probably the most player-friendly model available.