That's more or less what I'm highlighting here. It doesn't change the supply vs demand. The only ways to really increase it would be to give more rewards to the dps players to queue (not a good thing for the tanks/healers) or vice versa. Adding more slots in wont really change the issue of supply and demand either unless the dps numbers you can cram in are much, much higher.
That being said, I'd rather address the issue of leveling itself. If dungeons are the only 'quick' way to level, then we'd need to find other solutions that don't involve dungeons. Or, something more freeform that a group of 4 can do that doesn't require the typical 1/1/2 setup to do. The world is a very large place, and they can always find ways to do more with it.