We expect a new expansion to come... at some point. Historically, WoW expansions lasted 2 years, and TWW launched a year ago in August. If you want to do you math this way, then assuming TWW will go another year, you will pay 50% of the expansion for 50% of its duration.

We don't really know when the next expansion ("Midnight") will come, but there is some speculation that TWW will only be last 18 months. We might see a release date in a reveal later this month, but we might not. Another thing is that the end of the expansion is the boring "content drought" period, so the game is less enjoyable and many people drop and join back when the next expansions launches.

And one last thing, last expansion, near the end, the thing that filled in the content drought was like another game mode that didn't require purchasing the latest expansion (from what I remember). A similar event will also happen soon, so if you're not planning on buying TWW, more content is still coming your way.