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.)
I also think it's rather childish to throw a tantrum over being asked to do what a TV actor is hired to do - act the words the writer wrote. An actor has no right to expect to be consulted over the words that are written for him, because that's not his job. He's the actor, not the writer - let them write.
Yes, it might have been courteous to give him some notice - if they had known themselves, which they didn't - but it is in no way required. And it's rather petulant of him to imply they treated him badly by giving him an interesting script to work with.
If an actor wants to influence the direction of the story, he does it through his acting. That way the writers can pick up on what the actor is doing and work it into the character. But he does not doing it by refusing to learn his lines, standing in the corner with his arms folded and pouting his lips.