It's been said, admirable theory, and the distances from earth detail fits well.
However, I cannot fathom that after level 20 you'll never see the Fallen again, especially when they seem to be the closest to earth and the Wall.™ There may be certain areas that are "hard"areas, or the game gets harder as you level up, or enemies get buffs as you progress, or you get swarmed by more enemies, or you meet higher level enemies, etc etc etc. But more than likely as you progress, while new places will undoubtedly open up, you'll be able to return to all the old places and still be challenged.
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I know what you mean. The flaw in this difficulty design is that once you're a high level, the vast majority of enemies will be weak compared to you. That defeats the challenge, unless you're constantly pushing forwards. Like others have suggested, perhaps the enemies you fight will scale according to your level or an average level in your fireteam.