It seems like the assumption was that eventually, 17.5 would be the stock class, and 13.5 would be the 19T class. If the 27T stock is faster than 17.5 brushless, obviously not many people will bother running it at big events. You'll probably see attendance in the 13.5 class go quite a bit higher since it will be brushless only, and will be more of a true "spec" class than anything else.
What it sounds like, is that 19T is going to be replaced by brushless stock (13.5), and stock will remain 27T until it dies out due to lack of interest. Hopefully events will let go of brushed, and just make the classes 17.5, 13.5 and mod so we can move on.