All-aluminum forks, both aftermarket and OEM, are pretty much gone now and even aluminum steerers are rare. Cr-Mo steel, all-carbon or carbon with Cr-Mo steerers are the norm. I agree that your steerer failure was a rarity or Trek would certainly have been forced into a recall by now but George Hincapie lost a possible win at Paris-Roubaix when his Trek's aluminum steerer failed so it's not unheard of.