Keys are not even close to being a challenge. They are boring grind mechanic which nobody knows why they got in CI series. Maybe in CI4 and CI5 they could be described the way you did because they weren’t need to play. However CIU is built around using the keys and you can’t travel to different star systems (not that you need to as I said many times) and also you can’t buy anything (including lives that matter very much for gameplay).
As I said above CIU is built around the keys. So guaranteed prize after beating level is needed. Imagine someone playing a mission and not having a chance to grab any of they keys. He gained nothing from the mission and gets a shiny gold text “0 keys”. That’s outrageous.
Also I need to clarify that I’m not the first to suggest little reward for killing the boss. Here’s the first person (and even IA liked his idea) - Ideas wishes for the new game - #67 by TheGreatestRoman
EDIT: I’m scared. For 2 hours it’s showing that Gaunter is replying. I’m gonna get destroyed.