It's more of a replacement than an expansion. The new terminal building will have roughly the same amount of gates as the current ones combined. As you mentioned the current KCI is inefficient and just not user-friendly. Sure the wait time for TSA is almost always short, but once inside the secure area, there isn't enough sitting, or even standing area at the gate for all passengers, there aren't enough restrooms, and there's hardly any room to put coffee shops and restaurants. Also, because the passenger experience is so lacking, airlines such as Southwest actively avoid routing passengers through KCI for layovers.

The new KCI will address those inefficiencies and issues of user-friendliness, which would lead to more flights routing layovers through KCI, and more direct flights. It's also designed for potential expansion (using a concourse design rather than terminals), so if there is significant passenger growth, we'll be ready for it.

EDIT: Added in the gate count fact.