I dug out these two official responses by Dawn

Quote Originally Posted by Dawnseeker View Post
The reason Baal's health increases is to make sure he's around long enough for everyone to get a crack at him. We don't want the event lasting less than 15 minutes.
Quote Originally Posted by Dawnseeker View Post
We're not prepared to lift the cap, as there would be a real problem with high end player growth outpacing everything else in the game.

What would people thinking of having alternate bonuses for killing a tougher banish boss? Maybe a buff or boost of some kind temporarily, based on the HP of the boss killed?