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-...you are a gigantic female genitalia sanitizing apparatus.
The whole point of playing a game is having fun. If you achieve something and have to go through something tedious WEEKLY to keep it, then it's not fun. Remember Halo is supposed to appeal to a mass audience, and not no-life WoW PvPers if you get the reference.
If a person gets 50 and wants to play in that playlist again, they SHOULDNT be afraid to lose a rank nor be afraid to lose it if they don't play. People will come back if it's just for fun.
Plus, what do you do to people who play say Lone Wolves the most and didn't get their achievements? "Oh well looks like you're gonna have to lose skill while you try to get em."?
This is not a tournament, it's about bettering your game. Obviously, you can still lose skill, but if you reach a certain level you keep playing at that level until you improve enough to pass to the next plateau. If you're not that good, you'll just never get there, simple as that.