I've never watched B5 or any JMS show, but with a majority of TV shows, main characters aren't written off "at any time." Usually main characters only leave in either the first or last two episodes of a season, or in a midseason arc if the season is long enough to have such a thing. There are reasons for this, related to actor contracts and the fact that it makes the most sense to have major events like a character's departure to happen in episodes you know will have the most amount of viewers, so that they will see about it. Even on the rare occasions when a character is written out of a show at a different point in the season, everyone makes damned sure to hype the episode as a special event to make sure everyone's watching. Like when Rick left The Walking Dead, fifth episode of a sixteen episode season, but it was heavily promoted for two months prior that that would be his last episode.
Indeed, given actor departures tend to be announced several months in advance of when their last episode airs, meaning there is rarely ever a surprise character departure on television anymore and that 98% of the time, you can be guaranteed all the main cast will make it to the end. In fact, one of the reasons Hemmer's death flew under the radar and wasn't spoiled is because the actor is still under contract for the second season, meaning he is not in fact departing the show. He'll be playing a new character, though, but that's still not departing the show.
