We can’t have it have an infinite health bar, having a battle that could potentially run for too long has a number of it’s own issues. Most notably, it will timeout after a battle has gone on for too long.
As for a larger health bar generally, there has to be a limit at some point, having the overkill health generally around Gold reward total means each battle you have a chance to get to Gold rewards in one or as many battles as you have left. Designed with max personal goals in mind.
Thinking of the battle itself as a Kingdom/team effort and then Overkill being a personal battle to finish dealing enough damage to earn Gold rewards.
It is overkill, you have defeated the full health of the Dragon (it’s been “killed”) and now you are able to complete extra battles and deal additional damage.
I can definitely pass on the feedback to increase the amount of health available in overkill mode.