The ranking system in Reach means nothing, I completely agree. It only represents how long you have played the game. It let down so many fans of the game down. The AA's introduced a component to the game so significantly altered play along with slower player movement, lessened jump height, notably increased bloom, completely different primary rifle and overall weapon mechanics on both it and the sniper were significantly changed. The lack of a complete ranked play list was not reflective of Halo. What it had in its place, Arena, was so despised by players, no change would ever work to correct it. The game was so far removed from the it's successful predecessors, many fans felt it was not Halo, called it a stand alone game.
The games success in sales most likely fell on pre sales and and the success of the previous titles with the fans. Not long after it's release the numbers dropped to almost what thy hover at today. This is the true sign of what Reach meant to the Halo fan base.
No argument about other games, adapting or living in the past can really explain why so many left the game.