I probably should have started with a post replying to the OP.
The way I see it, Arthas is just too powerful lore wise. He was the Sarah kerrigan of Warcraft 3. I don’t play wow, but from what I’ve heard, the lich king expansion was the one where the original story ends and then after that they have like those extra seasons of TV where the writers aren’t allowed to end the story. The First Lich King expansion for Hearthstone was the most popular one ever, and the second one was the best performing in terms of player base increase over the past 4 years.
So there was never a question about whether there would be a death knight class as some kind of cash grab. It was only a question of when, and how well it would be done. Brode couldn’t stop it any more than he could body block a tsunami.