A full year to get to the end game content? Honestly, if your taking one full year to get there.....even casuals will have passed you up. If you haven't seen or beaten Twintania yet its either because you waited to even start the end game or your probably not good enough to do it period.

6 months is a very large, generous window on content. Making sure newer players can beat it when "new" content comes out is important for the health of the game and to attract new player, as well as keep everyone on a similar level playing field.

If content stayed the way it was for a year, and they add new stuff every 3 months.....you do realize how much it would take if you needed to catch someone up for your static or raid group?