The 5k cw cape was only made because jagex knew there already were some cw crazies. It makes absolutely no sense for the requirement of one minigame to be soooo much larger than for the other ones- it's just inconsistent. And as far as completing the game, i'd guess there are alot of people who don't believe anything below 5b xp is really completing the game, and while that would be a ridiculous requirement, so is 5k cw games(that's only one minigame, but takes as much time as 200m in a skill at 100k an hour, only 1-2 skills being under that currently). Anyone who argues for 5k cw games should also agree with 200m in agility, fishing, woodcutting or mining requirement for it to make any sense.
I agree with the above. There is already a reward for 5k so missing it out would mean you haven't completed the game.
At least castle wars is a lot more fun than getting 200m xp.