seems a bit unfair doesn't it? he blew up a few halos, stopped the covenant and the flood; hell he ended the conflict in this game by himself (not counting cortana). isn't it about time for a new rank?
build a man a fire and he will be warm for a day, but set a man on fire and he'll be warm for the rest of his life. Baka wa shinanakya naoranai.
I believe Master Chief Petty Officer is the highest attainable rank while still seeing combat. if his rank were higher he would just be giving orders and that defeats the purpose of the spartan program altogether.
The highest rank would be Admiral (unless you count the guys in ONI, who would definitely be even higher than that). But as was stated earlier in the topic, the Chief can't advance anymore. He's the man.
Unless he willingly requests a rank downgrade back to Boot and work his way up again, but come on, he wouldn't do that. Technically, he's a Chief Petty Officer, which is a normal rank that others can obtain, but through his strong leadership, the Chief earned leadership over all the other CPO's, including Mendez, the guy who trained him. Master Chief is the highest rank for Marine forces, and it's also literally a one-of-a-kind rank that no one else has. Who would give that up?
Also, the actual rank was created especially for him, and no other Spartan from any of the 4 Spartan programs came close to deserving it. If the Chief dies, the rank dies.
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