As characters in Killshot gain more experience, they take on more complex and dangerous jobs. While opponents surely get better, difficulty rolls and challenges generally do not. Characters improve and master such tasks to increase their chances of success, but in a game, this creates a situation where the only way to keep a challenge on par with the opponents is to increase the number of dice. If we simply add on more dice for the sake of adding on more dice, we're simply stating the original version of the game is broken.
So we have to increase the threat characters face; the repercussions for failure as their work brings them uncomfortably closer to dangerous or high-profile figures.