The garrison system is designed to give rewards and it's designed to give them without doing anything at all whatsoever once you have developed the utility already other than clicking on queues and queuing items and missions, and most current players have already developed their garrison.
But at the same time you can't just kill the garrison completely. People would want to do achievements in the future or get items it dropped. Unless they try to combat the obsession with achievement points and remove them completely, turn them into feats, but I very much doubt it.
So what are they going to do? Completely remove all rewards other than those giving achievements? Or those giving a mount? And if you do that, wouldn't it make the Garrisons dull? And isn't the time sink on Garrisons too big to be promoted to exist in future expansions to begin with?