You asked whether Trek didn't expect it to break, o simply was gambling that you wouldn't ride it that much. I expect that it's a bit of both, but mostly the first. Nothing lasts forever, so with enough use any frame will fail. Also, better bikes are built with weight being at a premium, so the better the bike, the less it'll be overbuilt.

A lot depends on how much you weigh, how you ride, and the types of roads. Unless you weigh over 200#s, or are riding absolutely miserable roads full of deep potholes, don't expect your frame to fail anytime soon.

Most frames survive until bent in crashes, and even after that. My guess is that the most likely thing to kill this frame will be damaging the right dropout by shifting the RD into the wheel.