I'm not aware of the success of Might and Magic VIII. But Heroes IV, despite being developed by a smaller team in a hurry with minimal involvement from JVC, sold well. And it certainly wasn't the game that killed 3DO. And despite all its flaws, Heroes IV is a very good game; it's just not quite what fans of Heroes II and III were expecting. Of course, the overall quality and development also played a role, as it was made by only part of the team that made the third game. But again, riding on the wave of the third game's success, the fourth game sold well.