There are people missing that particular savefile, such as myself. While it is simple to say deal with it or force people to replay many hours of gameplay, it is not really the ideal solution. The idea of any extra content is to want people to purchase and play it. You want to capture as much of your demographic as possible to maximize these profits. By alienating the fan base just so you can go "deal with it" is how companies lose consumers. Compared to more massive games, the demographic is already smaller. Is it really wise to blame the consumer and lose out on profits and future fans just because they potentially made a mistake? Entering the pit makes a single autosave file. It is a lot easier to lose it than some people let on.
While there was a warning at the pit, I do not recall there being a warning that you would also be locked out of an entire expansion. It already feels odd to finish the game, go back, not finish the game, go somewhere, and then maybe finish the game again depending on what the next expansion is, if there is one.