With the new SAB coming back out, a activity that everyone LOVES, why not make it stay and let us go in when ever we wont?
With you only doing 1 world per 6months about with a quest of only 4 worlds that is still 2yrs that SAb will be popular.
The ability to make more Worlds or added achievements for it, Monthly and dailies I dont see any reason why it MUST be a limit time activity.
I can understand you not wanting players to get “bored” of it, but I do truely believe new players and veteran players would highly enjoy this always being around.