I think it's naive to expect the writers to have planned everything out in advance for a five-to-seven year show. Of course they only came up with it at the last minute - that's what they do with ALL their ideas, every episode was basically come up with at the last minute, because that's how TV works. (Or at least it did at the time, in the 90s. Nowadays there tends to be more planning ahead, but still not generally more than a season at a time with any specificity.)
They kinda did that with Babylon 5. They had everyone's character arc already written 'in pencil' even including how they'd write out a character who's actor ever wanted to leave the show.