I was actually quite pleased with this series as a whole, even with this ending. It wasn't disappointing in the slightest.
It was made very clear that there could be no show down between calamity and anyone / everyone. It has to be his decision. If BS had attempted to stretch that decision out any further, it would have been painfully boring.
Adding larcener into the plot whilst trying to sort out prof was genius. It felt subtly obvious that he was more than a normal Epic. Through his character, I was able to form a picture of his inner struggle before the big, albeit short, revealand gain an understanding of his choice to leave.
In his choice to leave, I feel that he did save someone, well, a lot of someones. Epics now have the choice to be good, bad, or something in the middle without an overpowering evil influence over them. Those who would have been killed eventually by those who chose instead to be good would likely be very pleased with calamitys choice to leave, if they knew the consequences of him staying.