Well, I was thinking the very same thing, as I was rather surprised to find my Forge missing when I went to place it! I completed all the quests, collected the litter around my city 4-6 times a day, and used up all my event currency.
It seems an odd plan if that's going to be it, and extremely disappointing. One should always have some kind of recourse for set buildings, especially if the bar was set so high that doing everything right does not complete the set!