Well, first of all, welcome to the forums, weary traveler! Indeed the question has been raised several times.
It doesn’t matter how many times we ask about it, the consensus seems to be that the playerbase is too small to handle the split…especially with Gauntlet already doing exactly that. It is a problem, I guess, if the queue is split into thirds. The saving grace of Gauntlet is that it is not for everyone, and that it has rewards beyond allowing you complete the standard Daily Quests.
It has also been argued that a casual mode would bring in enough new players to compensate. While that may have been more true a few months ago, I think enough players have quit precisely because of the lack of variety in game modes to hinder this theory.
At this point, the only thing I can think of that would help Casual Mode survive is a combination of three initiatives happening all at once:
1: An substantially increased marketing investment
2: A powerful, fun, and balanced expansion
3: Re-introduce Casual Mode with more intelligent match-making algorithms


