I don't really understand how there were people "polluting" the higher ranks? How does someone grief or troll level 50 Halo? What is wrong with someone laboriously grinding up to a higher rank? If people don't have the skill to be a 50, or 48, or 45, or 40, it doesn't matter how they got to that numerical rank, their actual skill won't be able to keep them there.
Honestly the removal of 1-50 just sounds like it's being done so an imaginary player won't get their feelings hurt. Every possible type of ranking or tiered system has the chance for abuse, it's inherent in visualizing disparity. People want the best, even if they can't legitimately earn it. Full moons, 1-50, Onyx and Gold, doesn't matter what container you wrap it in, the Halo 4 ranking system will be subject to the same issues.
At least you know ranks have meaning and weight to them if someone is willing to boost to the top; there's nobody who's trying to break the system so they can have Onyx.